Methods and compositions for enhanced protein expression and purification

ABSTRACT

Methods for enhancing expression levels and secretion of heterologous fusion proteins in a host cell are disclosed.

The present application claims priority to U.S. Provisional Application 60/482,817 filed Jun. 26, 2003, the entire disclosure of which is incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to the field of recombinant gene expression and purification of expressed proteins. More specifically, the invention provides materials and methods which facilitate purification of heterologous proteins from a variety of different host species.

BACKGROUND OF THE INVENTION

Several publications and patent documents are cited throughout the specification in order to describe the state of the art to which this invention pertains. Full citations of these references can be found throughout the specification. Each of these citations is incorporated herein as though set forth in full.

Functional genomic studies have been hampered by the inability to uniformly express and purify biologically active proteins in heterologous expression systems (1). Despite the use of identical transcriptional and translational signals in a given expression vector, expressed protein levels have been observed to vary dramatically (2). For this reason, several strategies have been developed to express heterologous proteins in bacteria, yeast, mammalian and insect cells as gene-fusions (3-6).

The expression of heterologous genes in bacteria is by far the simplest and most inexpensive means available for research or commercial purposes. However, some heterologous gene products fail to attain their correct three-dimensional conformation in E. coli while others become sequestered in large insoluble aggregates or “inclusion bodies” when overproduced (7,8). Major denaturant-induced solubilization methods followed by removal of the denaturant under conditions that favor refolding are often required to produce a reasonable yield of the recombinant protein. Selection of ORFs for structural genomics projects has also shown that only about 20% of the genes expressed in E. coli render proteins that are soluble or correctly folded (9). These numbers are startlingly disappointing especially given that most scientists rely on E. coli for initial attempts to express gene products. Several gene fusion systems such as NUS A, maltose binding protein (MUP), glutathione S transferase (GST), and thioredoxin (TRX) have been developed (7). All of these systems have certain drawbacks, ranging from inefficient expression to inconsistent cleavage from desired structure.

Ubiquitin (Ub) and ubiquitin like proteins (Ubls) have been described in the literature (10-12). The SUMO system has also been characterized (13). SUMO (small-ubiquitin related modifier) is also known as Sentrin, SMT3, PIC1, GMP1 and UBL1. SUMO and the SUMO pathway are present throughout the eukaryotic kingdom and the proteins are highly conserved from yeast to humans (14). Yeast has only a single SUMO gene, which has also been termed SMT3. The yeast Smt3 gene is essential for viability (13). In contrast to yeast, three members of SUMO have been described in vertebrates: SUMO-1 and close homologues SUMO-2 and SUMO-3. Human SUMO-1, a 101 amino-acid polypeptide, shares 50% sequence identity with human SUMO-2/SUMO-3 (15). Yeast SUMO (SMT3) shares 47% sequence identity with mammalian SUMO-1. Although overall sequence homology between ubiquitin and SUMO is only 18%, structure determination by nuclear magnetic resonance (NMR) reveals that the two proteins possess a common three dimensional structure characterized by a tightly packed globular fold with β-sheets wrapped around one α-helix (16-17). Examination of the chaperoning properties of SUMO reveals that attachment of a tightly packed globular structure to the N-terminus of a protein can act as a nucleus for folding and protect the labile protein. All SUMO genes encode precursor proteins with a short C-terminal sequence that extends from the conserved C-terminal Gly-Gly motif (13). The extension sequence, 2-12 amino acids in length, is different in all cases. Cells contain potent SUMO proteases (known also as hydrolases) that remove the C-terminal extensions (18). The C-terminus of SUMO is conjugated to ε-amino groups of lysine residues of target proteins. The similarity sumoylation pathway enzymes to ubiquitin pathway enzymes is remarkable, given the different effects of these two protein modification pathways (19). Sumoylation of cellular proteins has been proposed to regulate nuclear transport, signal transduction, stress response, and cell cycle progression (20). It is very likely that SUMO chaperones translocation of proteins among various cell compartments, however, the precise mechanistic details of this function of SUMO are not known.

Other fusions promote solubility of partner proteins presumably due to their large size (e.g., NusA)(21). Fusion of proteins with glutathione S-transferase (GST)(22) or maltose binding protein (MBP)(23) has been proposed to enhance expression and yield of fusion partners. However, enhanced expression is not always observed when GST is used as GCST forms dimers and can retard protein solubility. Another problem with GST or other fusion systems is that the desired protein may have to be removed from the fusion. To circumvent this problem, protease sites, such as Factor Xa, thrombin, enterokinase or Tev protease sites are often engineered downstream of the fusion partner. Often in these cases, however, incomplete cleavage and inappropriate cleavage within the fusion protein is often observed (7). The present invention circumvents these problems.

SUMMARY OF THE INVENTION

In accordance with the present invention, compositions and methods for enhancing expression levels of a protein of interest are provided. An exemplary method comprises i) generating a construct encoding a fusion protein by operably linking a nucleic acid sequence encoding a carboxy-terminal domain of a Ubl molecule selected from the group consisting of SUMO, Ub, RUB, HUB, APG8, PG12, URM1, Ubi-L, FAT10 and ISG15 to a nucleic acid sequence encoding the protein of interest, and ii) introducing the nucleic acid encoding the fusion protein into a host cell. To purify the protein of interest, the method may further comprises: iii) lysing the cells expressing the fusion protein, iv) contacting the cellular lysate with an amino-terminal domain of the same said Ubl immobilized on a solid phase, optionally by a purification tag attached to the amino-terminal domain, thereby reconstituting the Ubl, and v) contacting the reconstituted, and now cleavable, Ubl with the appropriate Ubl protease to release the protein of interest from the fusion protein. According to another aspect of the invention, the reconstituted Ubl from step iv) is eluted from the column and is then contacted with the appropriate protease in solution to release the protein of interest from the fusion protein. According to another aspect of the invention, the reconstituted Ubl is formed in solution and then purified over the solid support. According to yet another aspect of the instant invention, Ubl protease inhibitors are included while the Ubl is being reconstituted and then removed prior to the addition of the Ubl protease.

In accordance with another aspect of the invention, compositions and methods for purifying a protein of interest are provided. An exemplary method comprises i) generating a construct encoding a fusion protein by operably linking a nucleic acid sequence encoding a carboxy-terminal domain of a Ubi molecule selected from the group consisting of SUMO, Ub, RUB, HUB, APG8, APG12, URM1, Ubi-L, FAT10 and ISG15 to a nucleic acid sequence encoding the protein of interest and to a nucleic acid encoding at least one purification tag, whereby the purification tag is added to the amino terminus of the carboxy-terminal domain which is attached to the amino-terminus of the protein of interest, ii) introducing the nucleic acid encoding the fusion protein into a host cell, iii) lysing the cells expressing the fusion protein, iv) optionally purifying the fusion protein by contacting the fusion protein with a solid support capable of binding at least one purification tag; v) contacting fusion protein containing solution with an amino-terminal domain of the same said Ubl immobilized on a solid phase, optionally by a purification tag attached to the amino-terminal domain, thereby reconstituting the Ubl, and vi) contacting the reconstituted, and now cleavable, Ubl with the appropriate Ubl protease to release the protein of interest from the fusion protein. According to another aspect of the invention, the reconstituted Ubl from step iv) is eluted from the column and is then contacted with the appropriate protease in solution to release the protein of interest from the fusion protein. According to another aspect of the invention, the reconstituted Ubi is formed in solution and then purified over the solid support. According to yet another aspect of the instant invention, Ubl protease inhibitors are included while the Ubl is being reconstituted and then removed prior to the addition of the Ubl protease. In yet another aspect of the invention, the Ubl may also comprise a purification tag to ensure its removal during the purification scheme.

In yet another embodiment of the invention, the molecule used to generate a fusion construct with the protein of interest is the carboxy-terminal part of SUMO (CTHS) encoded by a nucleic acid comprising the sequence of SEQ ID NO: 10. The molecule used for purification and cleavage of the fusion protein is the amino-terminal part of SUMO (NTHS) encoded by the nucleic acid comprising the sequence of SEQ ID NO: 6. The specific hydrolase (i.e. protease) used to cleave the reconstituted Ubl thereby releasing the protein of interest is a SUMO hydrolase encoded by a nucleic acid comprising the sequence of SEQ ID NO: 30.

In yet another embodiment of the invention, an exemplary method for generating a protein of interest having an altered amino terminus is provided. Such a method comprises altering the amino terminus of the protein of interest and performing the purification methods exemplified herein. In accordance with another aspect of the invention, the protein of interest with an altered amino-terminus may be produced in vivo by a method comprising: i) generating a construct encoding a fusion protein by operably linking a nucleic acid sequence encoding a carboxy-terminal domain of a Ubl molecule selected from the group consisting of SUMO, Ub, RUB, HUB, APG8, APG12, URM1, Ubi-L, FAT10 and ISG15 to a nucleic acid sequence encoding the protein of interest with an altered amino-terminus, ii) introducing the nucleic acid encoding the fusion protein into a host cell, iii) introducing a nucleic acid encoding an amino-terminal domain of the Ubl molecule into the host cell at a desired time, and iv) optionally introducing a nucleic acid encoding a Ubi specific protease into the host cell if cleavage of the reconstituted Ubl is insufficient.

In yet another embodiment of the invention, methods are provided for increasing the affinity between a carboxy-terminal domain of a ubiquitin-like (Ubl) molecule and a amino-terminal domain of a ubiquitin-like (Ubl) molecule. An exemplary method comprises operably linking a moiety to the carboxy-terminal domain and amino-terminal domain, wherein the attached moiety comprises an anti-parallel β-sheet structure, an anti-parallel a-helix structure, or negatively and positively charged amino acids. In accordance with another aspect of the invention, a method of increasing the affinity between a carboxy-terminal domain of a ubiquitin-like (Ubl) molecule and an amino-terminal domain of a ubiquitin-like (UbI) molecule is provided comprising inserting mutations into the carboxy-terminal domain and amino-terminal domain wherein the mutations increase the hydrophobicity between the two domains or the mutations introduce charged amino acids.

In yet another embodiment of the invention, kits are provided for performing the methods described herein. Such kits comprise a recombinant vector containing a nucleic acid sequence encoding a carboxy-terminal portion of a Ubl molecule selected from the group of SUMO, Ub, RUB, HUB, APG8, APG12, URM1, Ubi-L, FAT10 and ISG15 operably linked to a promoter suitable for expression in the desired host cell and a multiple cloning site suitable for cloning a nucleic acid encoding the protein of interest. The recombinant vector may also contain a nucleic acid sequence encoding for at least one purification tag. The kits may further comprise a preparation of a protease capable of cleaving a reconstituted Ubl molecule from the fusion protein optionally comprising a purification tag, a preparation of an amino-terminal portion of a Ubl molecule optionally comprising at least one purification tag, a recombinant vector containing a nucleic acid sequence encoding an amino-terminal portion of a Ubl molecule optionally comprising at least one purification tag, at least one solid phase capable of binding at least one purification tag, appropriate buffers including wash and cleavage buffers, instruction material, and frozen stocks of host cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing illustrating the conjugation pathways for ubiquitin and ubiquitin-like proteins (Ubls). An arrow in the “C-terminal hydrolase” column indicates the cleavage of the precursor proteins. Only enzymes previously described are provided. The failure to list a particular enzyme in a particular pathway does not preclude the existence of that enzyme.

FIG. 2 is a schematic representation of a cloning strategy used to generate CTHS fusion proteins. In this cloning strategy, the nucleic acid sequence encoding the protein to be expressed as a fusion with CTHS is amplified by PCR with primers that introduce a Bsa I site at the 5′ end and HindIII and BsaI sites at the 3′ end. The vector encoding for the CTHS contains, in this example, a HindIII site and an Esp31 site at the 3′ end of the CTHS encoding region. The PCR product is cleaved by Bsa I and the vector is cleaved by HindIII and an appropriate restriction enzyme (represented here by Esp31) allowing for insertion of the cleaved PCR product into the vector.

FIGS. 3A and 3B show three-dimensional structures of the SUMO (Smt3) protein (16,17) with the amino and carboxy termini and the boundary between the NTHS and CTHS indicated by arrows. FIGS. 3A and 3B differ only in that they are different views of the same protein. The backbone of the molecule is shown in stick formation with highly ordered structures shown as thick arrows (β-sheets) or coiled arrows (α-helices). The lower two panels show the structures of NTHS (left panel) wherein only the backbone of CTHS is shown and CTHS (right panel) wherein only the backbone of NTHS is shown. Exact sequences for NTHS and CTHS are depicted in FIGS. 14 and 15.

FIG. 4A is a schematic representation of the steps involved in a particular purification scheme using conventional affinity tags and matrices. FIG. 4B is a flow chart of the purification scheme. Briefly, the cleavable SUMO structure is reconstituted by complementing the purified CTHS fusion protein with purified NTHS. Upon reconstitution, the NTHS-CTHS structure becomes cleavable by Ulp1 protease and the protein of interest is released. Removal of the CTHS, NTHS and the protease is accomplished by chromatography on the appropriate affinity matrix. Examples of affinity tags include, without limitation, GST, 6xHis, MBP and HA epitope. See Example VI for a more detailed explanation of the presented purification schemes.

FIG. 5A shows a Coomassie stained gel that demonstrates that the attachment of the CTHS to the amino-terminus of target proteins increases the expression and/or enhances the solubility of the protein in E. coli. FIG. 5B is a graph of the intensity of fluorescence (in Arbitrary Units (AU) as measured in a Fluorscan Ascent FL fluorometer, LabSystems) in soluble fractions obtained from the cells expressing the various constructs. Green fluorescence protein (6xHis-GFP), 6xHis-CTHS-GFP, 6xHis-SUMO-GFP, GST-6xHis-CTHS-GFP, and GST-6xHis-SUMO-GFP fusions encoded in pET24d E. coli expression vectors were expressed in the E. coli Rosetta pLysS strain (Novagen). Expression was induced at 26° C. with 1 mM IPTG for two hours in LB medium. Lanes 1-6 represent total cellular protein and lanes 7-12 represent soluble proteins. The first lane of the gel contains molecular weight markers of the size indicated by the numbers to the left of the gel. Arrows indicate the migration position of the fusion proteins.

FIG. 6A is a Western blot of 6xHis-CTHS-GFP fusion proteins expressed in yeast cells demonstrating that the attachment of the CTHS to the amino-terminus of target protein increases expression of the protein and CTHS-GFP fusions are not cleaved in yeast. Yeast strain BJ1991 was transformed with one of the following plasmids: 6xHis-CTHS-GFP, 6xHis-SUMO-Met-GFP (cleavable), 6xHis-SUMO-Pro-GFP (uncleavable), or 6xHis-GFP. The plasmids express the fusion proteins under the control of a copper sulfate regulated promoter. Total cell extracts were prepared by boiling the cells in SDS-PAGE buffer and briefly sonicating the sample to reduce viscosity. 20 μg of the total yeast proteins were resolved on 15% SDS-PAGE minigel and analyzed by Western blot with a rabbit polyclonal antibody against GFP and a secondary HRP-conjugated antibody. FIG. 6B is a Western blot of 6xHis-CTHS-GFP fusion proteins expressed in insect cells demonstrating that CTHS-GFP fusions are not cleaved in insect cells (lanes 4 and 8). High Five cells infected with baculovirus carrying CIHS-GFP were harvested 72 hours post-infection. Total cell extracts were prepared by boiling the cells in SDS-PAGE buffer and briefly sonicating the sample to reduce viscosity. 20 μg of the total proteins were resolved on 15% SDS-PAGE minigel. Extracts from yeast cells (lanes 1-3 and 5-7, same as in FIG. 6A) were used as standards. Western blot was performed with anti-GFP (lanes 14) or anti-SUMO antibodies and a secondary HRP-conjugated antibody. GFP alone (lanes 1 and 4), CTHS-GFP (lanes 2,4, 6 and 8), and SUMO-proline-GFP (3 and 7) were loaded. Migration positions for each protein are indicated.

FIG. 7A is a Coomassie stained gel of 6xHis-CTHS-GFP fusion protein purified from E. coli that was incubated with purified 6xHis-NTHS (lanes 1-3), purified 15 - 6xHis-NTHS and SUMO hydrolase Ulp1(lanes 4-6), or SUMO hydrolase Ulp1 (lanes 7-9). Reaction products were resolved on SDS-PAGE and stained with Coomassie. Molecular mass standards are shown on the right. Notably, release of free GFP can be observed only when all the components are present in the reaction mix. FIG. 7B is a Coomassie stained gel of 6xHis-CTHS-GFP (lanes 3, 5, 7) and 6xHis-CTHS-1-GFP (lanes 4, 6, 8) fusion proteins purified from E. coli that were incubated with SUMO hydrolase Ulp1 (lanes 5-8) and purified 6xHis-NTHS (lanes 5-6) or 6xHis-NTHS-1 (lanes 7-8). Reaction products were resolved on SDS-PAGE and stained with Coomassie. Lanes 1 and 2 are controls of purified 6xHisNTHS and 6xHis-NTHS-1. Molecular mass standards are shown on the left. Notably, release of free GFP can be observed when any combination of CTHS and NTHS is used. Identification of the protein bands is at the right of the gel.

FIG. 8 depicts a Western blot of 6xHis-CTHS-GFP fusion protein that was partially purified from yeast cells and incubated alone (lane 1), with Ulp1 (lane 2), or with Ulp1 in the presence of NTHS (lane 3). Reaction products were resolved on SDS-PAGE transferred onto nitrocellulose and probed with anti-GFP antibodies. Lane 4 depicts the results from a reaction similar to lane 3 wherein the 6xHis-CTHS-GFP was purified from E. coli.

FIG. 9 is a Coomassie stained gel of purified GST-CTHS-GFP fusion proteins incubated alone (lane 1), with purified 6xHis-NTHS (lane 2), with Ulp1 (lane 3), or with purified 6xHis-NTHS and SUMO hydrolase Ulp1 (lanes 4 and 5). Lane 6 shows similar reaction as in lanes 4 and 5, but with purified 6xHis-CTHS-GFP. Reaction products were resolved on SDS-PAGE and stained with Coomassie. Positions of molecular mass standards are indicated on the left. Identification of the protein bands is at the right of the gel.

FIG. 10 is a Coomasssie stained gel depicting the purification of CTHS-GFP on immobilized NTHS or anti-SUMO IgGs. Total soluble proteins (1 mg) from E. coli expressing 6xHis-CTHS-GFP fusion were incubated with 15 μl (bed volume) of resins onto which specific proteins were covalently coupled. Samples were washed and resolved on 12% SDS-PAGE and stained with Coomassie. Molecular mass standards are shown on the right. The location of CTHS-GFP is indicated at the right.

FIG. 11 is a Coomassie stained gel of GST-6xHis-CTHS-GFP fusion protein purified from insect cells that was incubated alone (lanes 2-3), with purified 6xHis-NTHS (lanes 5-6), or purified 6xHis-NTHS and SUMO hydrolase Ulp1 (lanes 8-9). Reaction products were resolved on SDS-PAGE and stained with Coomassie. Molecular mass standards are shown on the left. Notably, release of free GFP can be observed only when all the components are present in the reaction mix. Control reactions with CTHS-GFP purified from E. coli are present in lanes 4 and 7. Identification of the protein bands is at the right of the gel.

FIG. 12 is the amino acid (SEQ ID NO: 1) and nucleotide (SEQ ID NO: 2) sequence of SUMO.

FIG. 13A is the amino acid (SEQ ID NO: 3) and nucleotide (SEQ ID NO: 4) sequences of 6xHis-SUMO as it appears in the pET-6xHis-SUMO plasmid. FIG. 13B is a map of the pET-6xHis-SUMO plasmid. FIG. 13C shows the regions (SEQ ID NO: 35) that flank the sequences of 6xHis-SUMO (SEQ ID NO: 3).

FIG. 14A shows the amino acid (SEQ ID NO: 5) and nucleotide (SEQ ID NO: 6) sequences of NTHS. FIG. 14B shows the amino acid sequence (SEQ ID NO: 7), nucleotide sequence (SEQ ID NO: 8) of 6xHis tagged NTHS as it appears in the pET-6xHis-NTHS plasmid.

FIG. 15A shows the amino acid (SEQ ID NO: 9) and nucleotide (SEQ ID NO: 10) sequences of CTHS. FIG. 15B shows the amino acid (SEQ ID NO: 11) and nucleotide (SEQ ID NO: 12) sequence of 6xHis-CTHS as it appears in the pET-6xHis-CTHS plasmid. FIG. 15C is a map of the pET-6xHis-CTHS plasmid. FIG. 15D shows the regions (SEQ ID NO: 36) that flank the sequences of 6xHis-CTHS (SEQ ID NO: 11).

FIG. 16 is the amino acid (SEQ ID NO: 13) and nucleotide (SEQ ID NO: 14) sequences of GFP (enhanced Green Fluorescent Protein).

FIG. 17 is the amino acid (SEQ ID NO: 15) and nucleotide (SEQ ID NO: 16) sequences of the 6xHis-GFP fusion protein as it appears in pET-6xHis-GFP, Yep and pFastBac plasmids.

FIG. 18 is the amino acid (SEQ ID NO: 17) and nucleotide (SEQ ID NO: 18) sequences of the 6xHis-SUMO-GFP fusion protein as it appears in pET-6xHis-SUMO-GFP, Yep and pFastBac plasmids.

FIG. 19 is the amino acid (SEQ ID NO: 19) and nucleotide (SEQ ID NO: 20) sequences of the 6xHis-CTHS-GFP fusion protein as it appears in pET-6xHis-CTHS-GFP, Yep and pFastBac plasmids.

FIG. 20 is the amino acid (SEQ ID NO: 21) and nucleotide (SEQ ID NO: 22) sequences of GST protein (glutathione transferase) followed by the recognition site for Tev protease.

FIG. 21 is the amino acid (SEQ ID NO: 23) and nucleotide (SEQ ID NO: 24) sequences of GST-6xHis-SUMO-GFP fusion as it appears in pET-GST-6xHis-SUMO-GFP, Yep and pFastBac plasmids.

FIG. 22 is the amino acid (SEQ ID NO: 25) and nucleotide (SEQ ID NO: 26) sequences of GST-6xHis-CTHS-GFP fusion as it appears in pET-GST-6xHis-CTHS-GFP, Yep and pFastBac plasmids.

FIG. 23 is the amino acid sequence (SEQ ID NO: 27) and nucleotide sequence of the Cys-6xHis-NTHS protein as it is encoded in pET-Cys-6xHis-NTHS.

FIG. 24 is the amino acid (SEQ ID NO: 29) and nucleotide (SEQ ID NO: 30) sequences of catalytic domain of Ulp1 SUMO protease (hydrolase).

FIG. 25 shows the amino acid (SEQ ID NO: 31) and nucleotide (SEQ ID NO: 32) sequences of CTHS-1; the amino acid (SEQ ID NO: 33) and nucleotide (SEQ ID NO: 34) sequence of 6xHis-CTHS-1 as it appears in the pET-6xHis-CTHS-1 plasmid; the amino acid (SEQ ID NO: 91) and nucleotide (SEQ ID NO: 92) sequences of NTHS-1; and the amino acid (SEQ ID NO: 93) and nucleotide (SEQ ID NO: 94) sequence of 6xHis-NTHS-1 as it appears in the pET-6xHis-NTHS-1 plasmid.

FIGS. 26A-C show the sequences of the carboxy and amino terminal portions of various Ubls. Two versions of each domain are given. The sequences of domains are extrapolated from the results obtained with SUMO protein and described in the examples.

FIGS. 27A-C show the sequences of the carboxy and amino terminal portions of various Ufds (ubiquitin folding domains). Two versions of each domain are given. The sequences of domains are deduced from solved 3D structures as described in Example VIII.

DETAILED DESCRIPTION OF THE INVENTION

There are a number of reasons for the lack of efficient recombinant protein expression in a host, including, for example, short half-life, improper folding, compartmentalization and codon bias. While the Human Genome project has successfully created a DNA “map” of the human genome, the development of protein expression technologies that function uniformly in different expression platforms and for all the protein motifs has not yet been achieved.

In accordance with the present invention, a method for improving the expression of a protein, particularly an unexpressed or poorly expressed protein, in both eukaryotes and prokaryotes is provided. The method comprises attaching a carboxy-terminal portion of a Ubl to the amino-terminus of the protein to be expressed (i.e. protein of interest). The Ubl family of proteins includes, but is not limited to, SUMO, Ub, Rub1, Hub1, ISG15, Ubi-L (MNSF), FAT10, Apg12, Apg8 and Urm1 (12) (see Table 1). A hallmark of all of these proteins, with the exception of APG12, and URM1, is that they are synthesized as precursors and processed by a hydrolase (or protease) at the very carboxy-terminus, usually a di-glycine motif, to generate a mature sequence. Notably, all of the Ubls share a common structure (24,25).

Methods employing fusion proteins comprised of full-length SUMO and other Ubls to obtain expression and purification has been previously described (U.S. application Ser. Nos. 10/338,411 and 10/389,640). Fusion proteins comprising full-length SUMO and a protein of interest remained intact only in E. coli. However, in yeast or insect cells the fusion proteins were immediately cleaved, except when proline was the amino-terminal residue of the protein of interest.

Provided herein are methods which allow for the expression of fusion proteins in both eukaryotes and prokaryotes as uncleaved entities. Importantly, this allows for any optionally attached affinity tag (i.e. purification tag) or any other protein or peptide that may be used for purification, detection or any other purpose to remain attached to the expressed protein of interest. Cleavage is only obtained when the Ubl is reconstituted by contacting the protein of interest-carboxy-terminal portion (domain) of the Ubl fusion protein with the amino-terminal portion of the Ubl and subsequently contacted with a specific hydrolase.

The present invention may also be utilized to generate proteins with novel N-termini in eukaryotic cells. The identity of the N-terminus of a protein has been proposed to control its half-life (the N-end Rule) (35). Many important biopharmaceuticals such as growth factors, chemokines, and other cellular proteins, require desired N-termini for therapeutic activity. It has not been possible to generate desired N-termini, as nature initiates translation from methionine. Furthermore, one or more amino acids may be removed from the N-terminus, other amino acids may be added and various covalent modifications may be introduced to the exposed N-termini (26). These modification systems are complex, present in both prokaryotes and eukaryotes and vary to some degree in specificity and performance (27). This means that each recombinantly produced protein that did not undergo in vitro processing by a protease of known specificity may have unexpected and/or heterogeneous N-termini. The full-length SUMO system, as described in U.S. application Ser. Nos. 10/338,411 and 10/389,640, offers a novel way to circumvent this problem for prokaryotic expression systems. According to the instant invention, any amino acid can be incorporated as the N-terminal residue of a protein of interest by altering the appropriate codon in the nucleic acid encoding for the protein of interest. The altered protein of interest is then attached to the carboxy-terminal portion of a Ubl, expressed in a desired cell, and purified as described herein through the reconstitution of the Ubl by contact with the amino-terminal portion of the Ubl and then cleaving the reconstituted Ubl with a specific protease. Alternatively, the amino-terminal domain of the Ubl may be expressed in vivo along with the fusion protein to produce the protein of interest so long as there is specific protease activity in the cell (i.e. native or provided exogenously). Every amino acid except for proline may be employed as the amino-terminal residue by these methods. To employ proline as the amino-terminal residue of the protein of interest, a fusion protein in which the cleavage site of the Ubl protease is followed by the amino acid sequence Met-Pro. Following reconstitution and cleavage with the appropriate protease, the Met-Pro amino-terminus of the protein of interest is further treated with a methionine aminopeptidase (e.g. from Pyrococcus furiosis; (28)) to remove methionine and leave proline as the amino-terminus.

Additionally, methods are provided herein which allow for the high-efficiency affinity purification of the fusion proteins through the exploitation of natural affinity between the partial sequences representing the carboxy-terminal (CTHS) and amino-terminal (NTHS) halves (domains) of SUMO. Importantly, the boundaries of CTHS and NTHS are flexible. Significantly, the instant invention provides two unique and functional forms of the carboxy-terminal domain of SUMO: CTHS (SEQ ID NO: 9) and CTHS-1 SEQ ID NO: 31). Therefore, the instant invention encompasses CTHS and CTHS-1, allelic and species variants, and any other modified CTHSs through either the lengthening of or shortening of the sequence by about 2, 4, 6, 8, 10, 15, and 20 amino acids (or the corresponding changes in nucleotides). Preferably, the modified CTHSs retain at least one of the activities of CTHS (SEQ ID NO: 9) (e.g., increased expression, binding of NTHS (SEQ ID NO: 5)) or a modified NTHS, and/or formation of a cleavable complex with NTHS or modified NTHS). Preferably, the modified CTHS contains the 2β-sheets, as depicted in FIG. 3, in their entirety or a significant portion thereof.

Similarly, modifications in the length of NTHS are encompassed in the present invention. Two unique and functional forms of the amino-terminal domain of SUMO are also provided: NTHS (SEQ ID NO: 5) and NTHS-1 (SEQ ID NO: 91). The instant invention encompasses NTHS and NTHS-1, allelic and species variants, and any other modified NTHSs through either the lengthening of or shortening of the sequence by about 2, 4, 6, 8, 10, 15, and 20 amino acids (or the corresponding changes in nucleotides). Preferably, the modified NTHSs retain at least one of the activities of NTHS (SEQ ID NO: 5) (e.g. the ability to bind CTHS (SEQ ID NO: 9) or a modified CTHS and/or formation of a cleavable complex with CTHS or modified CTHS). Preferably, the modified N7THS contains the 2β-sheets and α-helix, as depicted in FIG. 3, in their entirety or a significant portion thereof.

While any of the Ubls (i.e. their partial sequences, see FIG. 26) set forth in Table 1 may be utilized in the compositions and methods of the invention to I) enhance expression of heterologous fusion proteins of interest, 2) allow affinity purification of the fusion protein, and 3) reconstitution of the Ubl for cleavage, SUMO is exemplified in the gene fusion system provided herein. While any of the specific hydrolases (i.e. proteases) set forth in Table 2 may be utilized in the compositions and methods of the invention to remove CTHS (or other partial sequence of SUMO protein or other Ubl) from the proteins of interest, Ulp1 is exemplified in the gene fusion system provided herein. TABLE 1 Properties of Ubiquitin-like Proteins (Ubls) Knockout % UB Hydro- COOH Ubls Function phenotype Substrate Identity KDa lase Residues Ub Translocation not viable many 100 8.5 Numer- LRLR to ous GG proteasome SUMO Translocation not viable RanGap, 18 11.6 Ulp1/ GG (SMT3) to nucleus many others Ulp2 RUB1 Regulation of viable; Mostly 60 8.7 Den1/ GG (NEDD8) mitosis. non- cullins Ulp8 essential. HUB1 Cell viable; Sph1, Hbt1, 22 8.2 not YY polarization. deficient in cell polarity known mating. factors ISG15 Interferon αβ IFN, LPS PLCγ1, ˜30; 28 15.0 UBP43 LRLR (UCRP) responses, hypersensi- Stat1, many two (USP18) GG immune tivity; death others domains regulation APG12 Autophagy viable, Apg5 18 21.1 not FG defective in cleaved autophagy URM1 Unknown ts growth; unknown 20 11.0 not GG non- known essential. APG8 Autophagy viable; no phospatidyl- 18 13.6 Apg4/ FG (LC3) autophago- ethanol- Aut2 cytosis or amine sporulation FAT10 Interferon γ unknown unknown ˜41; 30 15 not GG responses two known domains Ubi-L immune unknown unknown 35 8.3 not GG (Fau) regulation known

TABLE 2 SUMO Hydrolases/Proteases Enzyme Properties Reference UB1-specific Protease 72 KDa. 621 residues Li and Hochstrasser, 1999 ULP1 Cleaves linear fusion and SUMO (29) isopeptides bonds. ULP2 (Yeast) 117 KDa, 1034 residues Li and Hochstrasser, 2000 Cleaves linear fusions and SUMO (30) isopeptide structures. SUMO-I C-Terminal 30Kda Suzuki, et al, 1999 Cleaves linear fusions and SUMO (31) isopeptide structures SUMO-I specific Protease 126 KDa 1112 residues Kim, et al, 2000 SUSP I (Human) Specific for SUMO-1 fusion but not (18) Smt3 fusion. Does not cleave isopeptide bond. Sentrin specific All of the SENP enzymes have Yeh, et al, 2000 Proteases (SENP) conserved C-terminal region with core (11) SENP1, SENP2 catalytic cysteine. Gong et al, 2000 SENP3, SENP4 The smallest SENP7 is 238 residues (32) SENP5, SENP6 and the largest SENP6 is 1112 SENP7 residues.

The split SUMO fusion system of the present invention has been successfully applied to express the green fluorescent protein (GFP) in E. coli and eukaryotic cells and to purify the final product from all systems. More specifically, the system allows for the: (1) enhancement of the expression of under-expressed proteins; (2) increasing of the solubility of proteins that are insoluble; (3) protecting proteins of interest from degradation by intracellular proteases by fusing partial Ubl sequences to their N-termini; (4) purification of the fusion proteins using immobilized complementing N-terminal part of the respective Ubl; (5) purification of the fusion proteins using immobilized specific antibodies against the Ubl or its part; (6) cleaving the fusion protein in vitro to efficiently generate authentic proteins; (7) generation of proteins with novel N-termini; and (8) cleavage of all fusion proteins with remarkable efficiency irrespective of the N-terminal sequence of the fused protein, using Ubl hydrolases such as SUMO hydrolase Ulp1.

Importantly, Ubl proteases are structure specific enzymes that recognize the entire structure of respective Ubl and not just several amino acid residues (16, 29, 30). On the contrary, most other known proteases, including the ones that are commonly used in recombinant protein processing, recognize small (48) and degenerate stretches of amino acid sequence and, as a consequence, often cleave within the protein of interest. The combination of properties of the Ubls (as noted hereinabove) with the specificity and robustness of the respective proteases creates a superior system for recombinant protein expression, purification and processing.

The ultimate fate of ubiquitinated or sumoylated proteins within a cell varies. A protein can be monoubiquitinated or polyubiquitinated. Monoubiquitination of proteins mainly regulates protein internalization (33). Ubiquitination primarily targets proteins to 26S proteasome for degradation (34). On the other hand, sumoylation of target proteins does not lead to degradation, but, rather, leads directly or indirectly to altered localization of proteins (10). There are about 50 deubiquitinating enzymes in human and 17 in yeast genomes, respectively. These enzymes cleave conjugated ubiquitin from target proteins as well as ubiquitin-ubiquitin and ubiquitin artificial-fusion proteins (35, 36). Yeast has two proteases, called Ulp1 and Ulp2, that remove SUMO from ε-amino groups of lysine as well from the linear SUMO-fusions (29, 30).

As compared to deubiquitinating enzymes, which are notoriously unstable (36, 37), Ulp1 is known to produce remarkable yields and to be highly stable and robust in its digestion properties. This dramatic difference between UBP and Ulp enzyme classes is not surprising since there is essentially no similarities between the two on the level of amino acid sequence or tertiary structure (16, 38). Since CTHS fusion can both enhance recombinant protein yield, provide means of protein purifications, and generate new N-termini, this technology provides an important tool for post-genomic biotechnology analyses.

The present invention also encompasses kits for use in effecting enhanced expression, secretion, purification, localization, and alteration of the amino terminus of a protein of interest. Such kits comprise a recombinant vector containing a nucleic acid sequence encoding a carboxy-terminal portion of a UBL molecule selected from the group of SUMO, Ub, RUB, HUB, APG8, APG12, URM1, Ubl-L, FAT10 and ISG15 operably linked to a promoter suitable for expression in the desired host cell and a multiple cloning site suitable for cloning a nucleic acid encoding the protein of interest in-frame with the nucleic acid sequence encoding the carboxy-terminal portion of the UBL molecule. The promoter is preferably a strong promoter and may be constitutive or regulated. Such promoters are well known in the art and include, but are not limited to, the promoters provided hereinbelow such as the ADH1, T7, and CUP-1 promoters.

The recombinant vector may also contain a nucleic acid sequence encoding at least one purification tag in-frame with the sequence encoding the carboxy-terminal portion of the Ubl molecule. Purification tags are well known in the art (see Sambrook et al., 2001, Molecular Cloning, Cold Spring Harbor Laboratory) and include, but are not limited to: polyhistidine tags, polyarginine tags, glutathione-S-transferase (GST), maltose binding protein (MBP), S-tag, influenza virus HA tag, thioredoxin, staphylococcal protein A tag, the FLAG™ epitope, AviTag epitope (for subsequent biotinylation), and the c-myc epitope. Materials and methods for the purification of fusion proteins via purification tags are also well known in the art (see Sambrook et al., Novagen catalog, 2002, examples hereinbelow). Reagents including, but not limited to, at least one solid support capable of binding the purification tag, lysis buffers, wash buffers, and elution buffers may also be included in the kits.

The kit may further comprise a composition comprising a preparation of an amino-terminal portion of the Ubl molecule. The kit may also include a recombinant vector containing a nucleic acid sequence encoding an amino-terminal portion of the Ubl molecule optionally operably linked to at least one purification tag. The kits may also further comprise a composition comprising at least one protease capable of cleaving the UBL molecule from the fusion protein, cleavage buffers, frozen stocks of host cells, and instruction manuals. The kits may also further comprise reagents for altering the nucleic acid encoding a protein of interest to generate amino termini which are different from those native to the wild-type protein. Methods for altering the nucleic acid are well known in the art and include, but are not limited to, site-directed mutagenesis and oligonucleotide-based site-directed mutagenesis (see BD Biosciences Catalog, 2001; Qiagen Catalog, 2001; Ausubel et al., eds., 1995, Current Protocols in Molecular Biology, John Wiley and Sons, Inc.).

As used herein, an “instructional material” includes a publication, a recording, a diagram, or any other medium of expression which can be used to communicate the usefulness of the composition of the invention for performing a method of the invention. The instructional material of the kit of the invention can, for example, be affixed to a container which contains a kit of the invention to be shipped together with a container which contains the kit. Alternatively, the instructional material can be shipped separately from the container with the intention that the instructional material and kit be used cooperatively by the recipient.

The following definitions are provided to facilitate an understanding of the present invention:

“Nucleic acid” or a “nucleic acid molecule” as used herein refers to any DNA or RNA molecule, either single or double stranded and, if single stranded, the molecule of its complementary-sequence in either linear or circular form. In discussing nucleic acid molecules, a sequence or structure of a particular nucleic acid molecule may be described herein according to the normal convention of providing the sequence in the 5′ to 3′ direction. With reference to nucleic acids of the invention, the term “isolated nucleic acid” is sometimes used. This term, when applied to DNA, refers to a DNA molecule that is separated from sequences with which it is immediately contiguous in the naturally occurring genome of the organism in which it originated. For example, an “isolated nucleic acid” may comprise a DNA molecule inserted into a vector, such as a plasmid or virus vector, or integrated into the genomic DNA of a prokaryotic or eukaryotic cell or host organism.

When applied to RNA, the term “isolated nucleic acid” may refer to an RNA molecule encoded by an isolated DNA molecule as defined above. Alternatively, the term may refer to an RNA molecule that has been sufficiently separated from other nucleic acids with which it would be associated in its natural state (i.e., in cells or tissues). An isolated nucleic acid (either DNA or RNA) may further represent a molecule produced directly by biological or synthetic means and separated from other components present during its production.

With respect to single stranded nucleic acids, particularly oligonucleotides, the term “specifically hybridizing” refers to the association between two single-stranded nucleotide molecules of sufficiently complementary sequence to permit such hybridization under pre-determined conditions generally used in the art (sometimes termed “substantially complementary”). In particular, the term refers to hybridization of an oligonucleotide with a substantially complementary sequence contained within a single-stranded DNA molecule of the invention, to the substantial exclusion of hybridization of the oligonucleotide with single-stranded nucleic acids of non-complementary sequence. Appropriate conditions enabling specific hybridization of single stranded nucleic acid molecules of varying complementarity are well known in the art.

For instance, one common formula for calculating the stringency conditions required to achieve hybridization between nucleic acid molecules of a specified sequence homology is set forth below (Sambrook et al., 1989): Tm=81.5° C.+16.6 Log[Na+]+0.41(% G+C)−0.63(% formamide)−600/#bp in duplex

As an illustration of the above formula, using [Na+]=[0.368] and 50% formamide, with GC content of 42% and an average probe size of 200 bases, the Tm is 57° C. The Tm of a DNA duplex decreases by 1-1.5° C. with every 1% decrease in homology. Thus, targets with greater than about 75% sequence identity would be observed using a hybridization temperature of 42° C. For example, hybridizations may be performed, according to the method of Sambrook et al., (supra) using a hybridization solution comprising: 5×SSC, 5× Denhardt's reagent, 1.0% SDS, 100 μg/ml denatured, fragmented salmon sperm DNA, 0.05% sodium pyrophosphate and up to 50% formamide. Hybridization is carried out at 37-42° C. for at least six hours. Following hybridization, filters are washed as follows: (1) 5 minutes at room temperature in 2×SSC and 1% SDS; (2) 15 minutes at room temperature in 2×SSC and 0.1% SDS; (3) 30 minutes-1 hour at 37° C. in 1×SSC and 1% SDS; (4) 2 hours at 42-65° in 1×SSC and 1% SDS, changing the solution every 30 minutes.

The stringency of the hybridization and wash depend primarily on the salt concentration and temperature of the solutions. In general, to maximize the rate of annealing of the probe with its target, the hybridization is usually carried out at salt and temperature conditions that are 20-25° C. below the calculated Tm of the hybrid. Wash conditions should be as stringent as possible for the degree of identity of the probe for the target. In general, wash conditions are selected to be approximately 12-20° C. below the Tm of the hybrid. In regards to the nucleic acids of the current invention, a moderate stringency hybridization is defined as hybridization in 6×SSC, 5× Denhardt's solution, 0.5% SDS and 100 μg/ml denatured salmon sperm DNA at 42° C., and washed in 2×SSC and 0.5% SDS at 55° C. for 15 minutes. A high stringency hybridization is defined as hybridization in 6×SSC, 5× Denhardt's solution, 0.5% SDS and 100 μg/ml denatured salmon sperm DNA at 42° C., and washed in 1×SSC and 0.5% SDS at 65° C. for 15 minutes. A very high stringency hybridization is defined as hybridization in 6×SSC, 5× Denhardt's solution, 0.5% SDS and 100 μg/ml denatured salmon sperm DNA at 42° C., and washed in 0.1×SSC and 0.5% SDS at 65° C. for 15 minutes.

The term “probe” as used herein refers to an oligonucleotide, polynucleotide or DNA molecule, whether occurring naturally as in a purified restriction enzyme digest or produced synthetically, which is capable of annealing with or specifically hybridizing to a nucleic acid with sequences complementary to the probe. A probe may be either single-stranded or double-stranded. The exact length of the probe will depend upon many factors, including temperature, source of probe and use of the method. For example, for diagnostic applications, depending on the complexity of the target sequence, the oligonucleotide probe typically contains 15-25 or more nucleotides, although it may contain fewer nucleotides. The probes herein are selected to be complementary to different strands of a particular target nucleic acid sequence. This means that the probes must be sufficiently complementary so as to be able to “specifically hybridize” or anneal with their respective target strands under a set of pre-determined conditions. Therefore, the probe sequence need not reflect the exact complementary sequence of the target. For example, a non-complementary nucleotide fragment may be attached to the 5′ or 3′ end of the probe, with the remainder of the probe sequence being complementary to the target strand. Alternatively, non-complementary bases or longer sequences can be interspersed into the probe, provided that the probe sequence has sufficient complementarity with the sequence of the target nucleic acid to anneal therewith specifically.

The term “primer” as used herein refers to a DNA oligonucleotide, either single-stranded or double-stranded, either derived from a biological system, generated by restriction enzyme digestion, or produced synthetically which, when placed in the proper environment, is able to functionally act as an initiator of template-dependent nucleic acid synthesis. When presented with an appropriate nucleic acid template, suitable nucleoside triphosphate precursors of nucleic acids, a polymerase enzyme, suitable cofactors and conditions such as a suitable temperature and pH, the primer may be extended at its 3′ terminus by the addition of nucleotides by the action of a polymerase or similar activity to yield a primer extension product. The primer may vary in length depending on the particular conditions and requirement of the application. For example, in diagnostic applications, the oligonucleotide primer is typically 15-25 or more nucleotides in length. The primer must be of sufficient complementarity to the desired template to prime the synthesis of the desired extension product, that is, to be able anneal with the desired template strand in a manner sufficient to provide the 3′ hydroxyl moiety of the primer in appropriate juxtaposition for use in the initiation of synthesis by a polymerase or similar enzyme. It is not required that the primer sequence represent an exact complement of the desired template. For example, a non-complementary nucleotide sequence may be attached to the 5′ end of an otherwise complementary primer. Alternatively, non-complementary bases may be interspersed within the oligonucleotide primer sequence, provided that the primer sequence has sufficient complementarity with the sequence of the desired template strand to functionally provide a template-primer complex for the synthesis of the extension product.

Polymerase chain reaction (PCR) has been described in U.S. Pat. Nos. 4,683,195, 4,800,195, and 4,965,188, the entire disclosures of which are incorporated by reference herein.

The terms “percent similarity”, “percent identity” and “percent homology” when referring to a particular sequence are used as set forth in the University of Wisconsin GCG software program.

The term “functional” as used herein implies that the nucleic or amino acid sequence is functional for the recited assay or purpose.

“Natural allelic variants”, “mutants” and “derivatives” of particular sequences of nucleic acids refer to nucleic acid sequences that are closely related to a particular sequence but which may possess, either naturally or by design, changes in sequence or structure. By closely related, it is meant that at least about 75%, but often, more than 90%, of the nucleotides of the sequence match over the defined length of the nucleic acid sequence referred to using a specific SEQ ID NO. Changes or differences in nucleotide sequence between closely related nucleic acid sequences may represent nucleotide changes in the sequence that arise during the course of normal replication or duplication in nature of the particular nucleic acid sequence. Other changes may be specifically designed and introduced into the sequence for specific purposes, such as to change an amino acid codon or sequence in a regulatory region of the nucleic acid. Such specific changes may be made in vitro using a variety of mutagenesis techniques or produced in a host organism placed under particular selection conditions that induce or select for the changes. Such sequence variants generated specifically may be referred to as “mutants” or “derivatives” of the original sequence.

The phrase “consisting essentially of” when referring to a particular nucleotide or amino acid means a sequence having the properties of a given SEQ ID NO:. For example, when used in reference to an amino acid sequence, the phrase includes the sequence per se and molecular modifications that would not affect the basic and novel characteristics of the sequence. The term “promoters” or “promoter” as used herein can refer to a DNA sequence that is located adjacent to a DNA sequence that encodes a recombinant product. A promoter is preferably linked operatively to an adjacent DNA sequence. A promoter typically increases an amount of recombinant product expressed from a DNA sequence as compared to an amount of the expressed recombinant product when no promoter exists. A promoter from one organism can be utilized to enhance recombinant product expression from a DNA sequence that originates from another organism. For example, a vertebrate promoter may be used for the expression of jellyfish GFP in vertebrates. In addition, one promoter element can increase an amount of recombinant products expressed for multiple DNA sequences attached in tandem. Hence, one promoter element can enhance the expression of one or more recombinant products. Multiple promoter elements are well-known to persons of ordinary skill in the art.

The term “enhancers” or “enhancer” as used herein can refer to a DNA sequence that is located adjacent to the DNA sequence that encodes a recombinant product. Enhancer elements are typically located upstream of a promoter element or can be located downstream of or within a coding DNA sequence (e.g., a DNA sequence transcribed or translated into a recombinant product or products). Hence, an enhancer element can be located 100 base pairs, 200 base pairs, or 300 or more base pairs upstream or downstream of a DNA sequence that encodes recombinant product. Enhancer elements can increase an amount of recombinant product expressed from a DNA sequence above increased expression afforded by a promoter element. Multiple enhancer elements are readily available to persons of ordinary skill in the art.

The terms “transfected” and “transfection” as used herein refer to methods of delivering exogenous DNA into a cell. These methods involve a variety of techniques, such as treating cells with high concentrations of salt, an electric field, liposomes, polycationic micelles, or detergent, to render a host cell outer membrane or wall permeable to nucleic acid molecules of interest. These specified methods are not limiting and the invention relates to any transformation technique well known to a person of ordinary skill in the art.

A “replicon” is any genetic element, for example, a plasmid, cosmid, bacmid, phage or virus, that is capable of replication largely under its own control. A replicon may be either RNA or DNA and may be single or double stranded.

A “vector” is a replicon, such as a plasmid, cosmid, bacmid, phage or virus, to which another genetic sequence or element (either DNA or RNA) may be lo attached so as to bring about the replication of the attached sequence or element.

An “expression operon” refers to a nucleic acid segment that may possess transcriptional and translational control sequences, such as promoters, enhancers, translational start signals (e.g., ATG or AUG codons), polyadenylation signals, terminators, and the like, and which facilitate the expression of a polypeptide coding sequence in a host cell or organism.

The term “oligonucleotide,” as used herein refers to sequences, primers and probes of the present invention, and is defined as a nucleic acid molecule comprised of two or more ribo- or deoxyribonucleotides, preferably more than three. The exact size of the oligonucleotide will depend on various factors and on the particular application and use of the oligonucleotide.

The term “substantially pure” refers to a preparation comprising at least 50-60% by weight of a given material (e.g., nucleic acid, oligonucleotide, protein, etc.). More preferably, the preparation comprises at least 75% by weight, and most preferably 90-95% by weight of the given compound. Purity is measured by methods appropriate for the given compound (e.g. chromatographic methods, agarose or polyacrylamide gel electrophoresis, HPLC analysis, and the like).

The term “gene” refers to a nucleic acid comprising an open reading frame encoding a polypeptide, including both exon and (optionally) intron sequences. The nucleic acid may also optionally include non-coding sequences such as promoter or enhancer sequences. The term “intron” refers to a DNA sequence present in a given gene that is not translated into protein and is generally found between exons.

The phrase “operably linked,” as used herein, may refer to a nucleic acid sequence placed into a functional relationship with another nucleic acid sequence. Examples of nucleic acid sequences that may be operably linked include, without limitation, promoters, cleavage sites, purification tags, transcription terminators, enhancers or activators and heterologous genes which when transcribed and, if appropriate to, translated will produce a functional product such as a protein, ribozyme or RNA molecule. The phrase “operably linked” may also, for example, refer to a nucleic acid sequence encoding a protein of interest placed in functional relationship with a nucleic acid encoding the carboxy-terminal domain of a Ubl such that the catalytic cleavage activity of the carboxy-terminal domain of a Ubl in proteinaceous form leads to the release of the protein of interest.

The phrase “solid support” refers to any solid surface including, without limitation, any chip (for example, silica-based, glass, or gold chip), glass slide, membrane, bead, solid particle (for example, agarose, sepharose, polystyrene or magnetic bead), column (or column material), test tube, or microtiter dish.

The phrases “affinity tag,” “purification tag,” and “epitope tag” may all refer to tags that can be used to effect the purification of a protein of interest. Purification/affinity/epitope tags are well known in the art (see Sambrook et al., 2001, Molecular Cloning, Cold Spring Harbor Laboratory) and include, but are not limited to: polyhistidine tags (e.g. 6xHis), polyarginine tags, glutathione-S-transferase (GST), maltose binding protein (MBP), S-tag, influenza virus HA tag, thioredoxin, staphylococcal protein A tag, the FLAG™ epitope, AviTag epitope (for subsequent biotinylation), dihydrofolate reductase (DHFR), an antibody epitope (e.g., a sequence of amino acids recognized and bound by an antibody), and the c-myc epitope.

The materials and methods set forth below are provided to facilitate the practice of the present invention.

Design and Construction of E. coli Expression Vectors

The original vector backbone was developed using pET 24d vector from Novagen (see FIGS. 13B and 15C). pET24d uses a T7 promoter system that is inducible with IPTG. The vector has a kanamycin selection marker and does not contain any translation terminator. Similar vectors using pET22 backbone, which carries a gene for ampicillin resistance, was also produced.

Construction of 6xHis-SUMO-GFP Cleavable and Uncleavable Fusions

An amino-terminal 6xHis-tagged SUMO fusion vector was constructed as follows. A PCR product was generated with the primers 5′CCATGGGTCATCACCATCATCATCACGGGTCGGACTCAGAAGTCAATC AA-3′ (SEQ ID NO: 69) and 5′-GGATCCGGTCTCAACCTCCAATC TGTTCGCGGTGAG-3′ (SEQ ID NO: 70) using a modified SUMO (Smt3) gene as a template ((39) kind gift of Erica Johnson). Specifically, the SUMO (SEQ ID NO: 2) used throughout the examples provided herein contains an adenine nucleotide at position 229 in place of a guanine nucleotide reported in the reported SUMO sequence (GenBank accession number U27233). This alteration produces an alanine instead of a threonine in the encoded amino acid sequence (compare SEQ ID NO: 1 with GenBank Accession No. Q12306). Although the following examples employ the altered version of SUMO, the instant invention includes the utilization of the “wild-type” SUMO as provided in the above listed GenBank Accession Nos.

The PCR fragment was double digested with Nco I and Bam HI, and then ligated into pET24d, which had been similarly digested. The eGFP sequence was amplified with the primers 5′-GGTCTCAAGGTATGGTGAGCAAGGGCGAGGAGC-3′ (SEQ ID NO: 71) and 5′-AAGCTTATTACTTGTACAGCTCGT CCATGCC-3′(SEQ ID NO: 72). To generate an uncleavable variant of SUMO-GFP fusion, a primer with proline codon in place of methionine (all shown in bold) was used for PCR amplification: 5′-GGTCTCAAGGTCCCGTGAGCAAGGGCGAGGAGC-3′ (SEQ ID NO: 73). The PCR products were purified and double digested with Bsa I and Hind III, these were then ligated into the pET24d-6xHis-SUMO vector which had been similarly digested. Plasmids were sequenced to confirm the presence of correct sequence in each.

Construction of pET-6xHis-NTHS and pET-Cys-6xHis-NTHS Plasmids

The pET-xHis-NTHS plasmid (FIG. 14) was constructed by inverse PCR using pET-6xHis-SUMO-GFP plasmid (FIG. 18). The primers 5′-TTTTTTAAGCTTGCGGCCGCACTCG-3′ (SEQ ID NO: 74) and 5′-TTITTTAAGCTTATTTAGCGAACGCTTCCATC-3′(SEQ ID NO: 75) were used to amplify the plasmid and to delete the C-terminal half of SUMO gene and the GFP gene. The amplified product was digested with Hind III restriction endonuclease and self-ligated. To introduce the cysteine residue close to the N-terminus of NTHS (FIG. 23, SEQ ID: 27) so that the covalent coupling to solid support (e.g. Sulfo-link resin, Pierce; and supports possessing thiopropyl, iodopropyl, and other thiol reactive groups) can be achieved, primers of the following sequences 5′-GATATACCATGGGTTGCCATCACCATC-3′ (SEQ ID NO: 76) and 5′-GATGGCAACCCATGGTATATCTCC-3′(SEQ ID NO: 77) were used for inverse PCR. The template was pET-6xHis-NTHS (FIG. 14). After completion of PCR, the product was purified, digested with Nco I and self ligated.

Construction of pET-6xHis-CTHS-GFP, pET-6xHis-E-CTHS-GFP and pET-S-6xHis-CTHS-GFP Plasmids

The pET-6xHis-CTHS-GFP plasmid was constructed by inverse PCR using pET-6xHis-SUMO-GPP plasmid. The primers 5′-TTTTTTGGTTCTCGTCATCATCACAAAAGACAGGGTAAGGAAATG-3′(SEQ ID NO: 78) and 5′-TTTTTTGGTCTCGATGATGGTGATGACCCATGG-3′ (SEQ ID NO: 79) were used to amplify the plasmid and to delete the N-terminal part of SUMO gene. The amplified product was digested with Bsa I restriction endonuclease and self-ligated.

To replace lysine with the glutamic acid residue (E in single letter code, hence the name of the plasmid) after 6xHis-tag, so that the 6xHis-tag can be protected from clipping by endopeptidases the PCR and cloning was performed as above except that primer of the following sequence 5′-TTTTTTGGTCTCGTCATCATCACGAAAGACAGGGTAAGGAAATG-3′ (SEQ ID NO: 80) was used instead of (SEQ ID NO: 78).

To introduce the part of S-tag sequence (40) so that the 6xHis-tag can be protected from clipping by exopeptidases the following two oligonucleotides were designed in such a way that upon annealing they form two Nco I compatible overhangs: (SEQ ID NO:81) 5′-CATGGAAACCGCTGCTGCTAAATTCGAACGCCAGCA-3′ and (SEQ ID NO:82) 5′-CATGTGCTGGCGTTCGAATTTAGCAGCAGCGGTTTC-3′. These were phosphorylated by a T4 DNA kinase annealed and cloned into Nco I digested pET-6xHis-CTHS-GFP. Construction of pET-GST-6xHis-SUMO-GFP and pET-GST-6xHis-CTHS GFP Plasmids

The pET-GST-6xHis-SUMO-GFP plasmid (FIG. 21, SEQ ID: 24) was constructed by inserting a PCR amplified GST sequence (FIG. 20, SEQ ID: 22) into Nco I site of pET-6xHis-SUMO-GFP plasmid. GST sequence was amplified from pET-GST plasmid using the primers 5′-TTTTTTCGTCTCCCATGTCCCCTATACTAGGTTAATTG-3′ (SEQ ID: 83) and 5′-TTTTTTTCCATGGCACCTTGAAAATAAAGAT-3′ (SEQ ID: 84) and the PCR product was digested with Esp3 I and Nco I. pET-GST-6xHis-CTHS-GFP (FIG. 22, SEQ ID: 26) was produced by inverse PCR using pET-GST-6xHis-SUMO-GFP plasmid as a template. The primers 5′-TTTTTTGTCTCGTCATCATCACAAAAGACAGGGTAAGGAAATG-3′ (SEQ ID NO: 78) and 5′-TTTTTTGGTCTCGATGATGGTGATGACCCATGG-3′ (SEQ ID NO: 79) were used to amplify the plasmid and to delete the N-terminal part of SUMO gene. The amplified product was digested with Bsa I restriction endonuclease and self-ligated.

Ulp1 Plasmid, Expression and Purification of SUMO Protease

The catalytic domain of SUMO hydrolase/protease Ulp(403-621)p (16, 29) was PCR amplified from yeast genomic DNA using the primers 5′-TTTTTTTCCATGGGACTTGTTCCTGAATTAAATGAA-3′ (SEQ ID: 85) and 5′-TTTTTTCTCGAGTTTTAAAGCGTCGGTTAAAATCAA-3′ (SEQ ID: 86). PCR product was digested with Nco I and Xho I and cloned into pET24d vector digested with Nco I and Xho I. The resulting clone carried a catalytic domain of Ulp1 in frame with C-terminal His-tag. The enzyme was expressed in Rosetta(DE3) pLysS (Novagen). The recombinant protein was purified using Ni-NTA agarose (Qiagen) and extensively dialyzed against 20 mM Tris-HCl pH 8.0, 150 mM NaCl and 5 mM β-mercaptoethanol. Prior to storage at −80° C. the enzyme preparations were adjusted to 50% glycerol.

Design and Construction of Yeast Vectors

Saccharomyces cerevisiae has been used as a model for experiments involving yeast. All of the expression vectors for these studies were designed on pYep that is a multicopy yeast vector with tryptophan as a selectable marker, 2μ as an origin of replication and a copper induced promoter that drives expression of cloned genes of interest (41). Fusion constructs (Nco I-Xho I fragments excised from E. coli vectors) were directly recloned into pYep digested with Nco I and Xho I.

Design and Construction of Recombinant Baculovirus and Expression of Fusions in Insect Cells

To demonstrate that CTHS-GFP fusion is produced as uncleaved protein which can be subsequently cleaved in vitro by contacting with NTHS and Ulp1 we used pFastBac vector (Invitrogen). Fusion construct (Nco I-Xho I fragments excised from E. coli vectors pET-GST-6xHis-CTHS-GFP, FIG. 22, SEQ ID: 26) was directly recloned into pFastBac digested with Nco I and Xho I.

Analysis of Proteins from Insect Cell Compartments: Preparation of Baculovirus Stocks and Cell Growth

Transfer vector constructs based on the pFastbac 1 shuttle plasmid (Invitrogen, Inc.) were transposed in DH10Bac E. coli competent cells to transfer the GST-6xHis-CTHS-GFP sequence into recombinant virus DNA by site-specific integration. After alkaline lysis of transformed (white colonies) of E. coli cells, which contain recombinant virus (bacmid) DNA, and extraction of the recombinant bacmid DNA, the bacmid DNA was used to transfect Spodoptera frugiperda (Sf9) insect cells, in which virus replication occurs. The virus was then amplified to produce passage 2 (for long-term storage) and passage 3 virus (for working) stocks by infection of fresh Sf9 cell cultures and used directly to infect cells for fusion protein expression. Virus infectivity (pfu/ml) was determined by titration in Sf9 cells using the BacPAK™ Rapid Titer Kit (BD Sciences Clontech, Inc.). A 50 ml culture of Hi-Five cells at concentration of 1×10⁶ cells/ml, was infected with recombinant virus at MOI=5 in Express Five media (serum free media). The cells were grown in 100 ml spinner flask at 27° C. Every 24 hours, cell viability was determined by trypan blue and cell counting 0.5 ml of the suspension

The presence of recombinant protein in cells and media was ascertained by SDS-PAGE and Western blotting of supernatant and cell pellets.

S. cerevisiae Growth and Protein Expression

Yeast cultures were grown in rich medium. Standard yeast and E. coli media were prepared as described (42). The yeast strain BJ 1991 was used as a host. Yeast transformation was performed according to published procedures (43). Yeast transformants with autonomously replicating plasmids were maintained in yeast selective media. The 6xHis-CTHS-GFP (cleavable or uncleavable variants) as well as S-6xHis-CTHS-GFP and 6xHis-E-CTHS-GFP fusions were expressed under the regulation of copper metallothioneine (CUP1) promoter in 2 μm multicopy plasmids with the TRP selective marker.

Yeast cells were transformed with appropriate expression vectors, and single colonies were grown in synthetic media minus the selectable marker. For each protein, at least two single colonies were independently analyzed for protein expression. Cells were grown in 5 ml culture overnight and, in the morning, the culture was diluted to an OD₆₀₀=0.5 with fresh medium. Culture was allowed to grow until OD₆₀₀=2.0 and copper sulfate was added to 100 μM. The culture was induced for three hours. Cells were pelleted at 2000×g for 5 minutes, washed with 10 mM Tris-EDTA buffer pH 7.5. Cell pellet was suspended in SDS-PAGE buffer and boiled for 5 min and briefly sonicated to shear the DNA. The suspension was centrifuged, and 10-20 μl aliquots were run on 12% SDS-PAGE.

E. coli Growth and Protein Expression

Protein expression studies were carried out in the Rosetta bacterial strain (Novagen). This strain is derived from the lambda DE3 lysogen strain and carries a chromosomal copy of the IPTG inducible T7 RNA polymerase along with tRNAs on a pACYC based plasmid. Cultures were grown in L3 as well as minimal media and at growth temperatures of 37° C. with 100 μg/mL Kanamycin and 30 μg/mL chloramphenicol. The culture was diluted 50 fold and grown to mid log (OD at 600 nm=0.5-0.7), at which time the temperature was reduced to 26° C. and the culture was induced with 1 mM IPTG. Induction was allowed to proceed for 2-5 hrs. To analyze protein induction in total cells, SDS-PAGE buffer was added and the protein was analyzed following SDS-PAGE and staining with Coomassie blue.

Separation of Soluble and Insoluble Fractions

E. coli were harvested by mild centrifugation and washed once with PBS buffer. Cells were resuspended in 4 ml of PBS and ruptured by several pulses of sonication. Unbroken cells were removed by mild centrifugation (5 min at 1500×g) and supernatants were sonicated again to ensure complete cell lysis. An aliquot (5 μl) was mixed with 2% SDS to ensure that no viscosity is detected owing to lysis of unbroken cells. After ensuring that no unbroken cells remained in the lysate, insoluble material consisting of cell walls, inclusion bodies and membrane fragments was sedimented by centrifugation (18,000×g for 10 min). The supernatant was considered the “soluble fraction”.

The pellets were washed from any remaining soluble proteins, lipids and peptidoglycan as follows. Pellets were resuspended in 600 μl of PBS and to the suspensions 600 μl of solution containing 3 M urea and 1% Triton X100 was added. The suspension was briefly vortexed and insoluble material was collected by centrifugation as above. The PBS/Urea/Triton wash was repeated two more times to ensure complete removal of soluble proteins. The washed pellets, designated as “insoluble fraction,” consisted primarily of inclusion bodies formed by over expressed proteins. Approximately 10 μg of protein from each fraction was resolved on 12% SDS-PAGE minigels and stained with Coomassie Brilliant Blue.

Fluorescence (GFP Activity) Assessment

GFP fluorescence (9) was measured in soluble fractions (approx. 0.1 mg of soluble protein in a final volume of 40 μl) using Fluoroscan Accent FL fluorimeter (LabSystems; Helsinki, Finland) with Excitation 485 nm/ Emission 510 nm filter set is with the exposure set to 0.4 sec. The data are presented in Arbitrary Units (AU).

Western Blotting

Twenty μg of total yeast protein per lane were resolved on 12% SDS-PAGE minigel and electro-blotted onto nitrocellulose membranes by standard methods. Prior to blocking and incubation with the antibodies the membranes were stained with Ponceau S solution (Sigma) to ensure equal protein amount in the samples. Membranes were blocked with 5% milk in TTBS buffer and incubated with rabbit anti-GFP antibodies (Clontech, cat no. 8367) at 1:500 dilution overnight at 4° C. Secondary HRP-conjugated antibodies were from Amersham.

Protein Purification and In Vitro Cleavage with Ulp1.

The recombinant proteins from soluble fractions of the lysates were purified using Ni-NTA agarose (Qiagen) using procedures recommended by the manufacturer and further detailed in EXAMPLE VI. Cleavage of the protein was performed as detailed in EXAMPLE VI.

The following examples are provided to illustrate various embodiments of the present invention. SUMO is exemplified in the following examples, however the other Ubls may be employed instead. The examples are illustrative and are not intended to limit the invention in any way.

EXAMPLE I Attachment of Carboxy-Terminal Half of SUMO (CTHS) to N-Terminus of GFP Enhances the Expression of the Protein in E. coli

The design and construction of all the pET vectors expressing GFP has been described above. The DNA sequences, accession numbers of the SUMO, CTHS, GST, GFP, the fusion constructs, plasmid maps, and translation frames are shown in FIGS. 12-24. FIG. 5 shows the expression pattern of 6xHis-GFP, 6xHis-SUMO-GFP, 6xHis-CTHS-GFP, GST-6xHis-SUMO-GFP, and GST-6xHis-CTHS-GFP. Induced E. coli cells were ruptured by sonication and soluble proteins were analyzed on SDS-polyacrylamide gels. The stained gel shows (FIG. 5A) that the fusions were soluble. Un-fused GFP is generally poorly expressed in E. coli. The data show that SUMO and CTHS with and without attachment of GST enhance the expression level of GFP to varying degrees. FIG. 5B shows GFP fluorescence in approximately 0.1 mg of soluble protein in a final volume of 40 μl using Fluoroscan Accent FL fluorometer (LabSystems). The data are presented in Arbitrary Units (AU) and show that SUMO, CTHS and their fusions with GST produced GFP protein that was able to fluoresce and, thus, folded correctly (9). Notably, CTHS fusion proteins consistently and reproducibly produced more GFP than the fusions with full-length SUMO. In addition to data presented here we constructed a shorter version of CTHS (CTHS-1, FIG. 25, SEQ IDs: 31-34). Analysis of the expressed CTHS-1-GFP fusion demonstrated that expression levels similar to CTHS are obtained indicating that the boundary between CTES and NTHS is relatively flexible and may be varied according to specific application.

Overall, these results show that in bacteria, fusion of CTHS to GFP increases the level of expression 10-40 fold (based on protein band appearance on the SDS-PAGE) or 3-12 fold (based on fluorescence comparison). The difference in these estimates is explained by effective quenching of GFP fluorescence in solutions with high concentration of proteins such as cell lysates.

EXAMPLE II Attachment of Carboxy-Terminal Half of SUMO (CTHS) to the N-Terminus of GFP Enhances the Expression of the Protein in Yeast Without Cleavage

The design and construction of all the Yep-based plasmids expressing various fusions of GFP and the methods for protein expression analysis have been described in detail above. Briefly, the constructs initially made in the pET24 vector were excised and recloned into the YEp vector. The transformants were selected on medium lacking tryptophan. Expression was induced for three hours and the proteins were analyzed by western blot with anti-GFP antibodies.

FIG. 6 shows a representative blot of GFP-fusions produced by yeast when expression is performed in YPD nutrient rich medium. There was a 10-20 fold increase in expression of GFP following fusion with CTHS. However, a nearly 50-fold increase in GFP production has been observed when cells were induced with 100 μM copper sulfate in synthetic medium. This variation in fold-increase is likely due to chelation of Cu ions by various compounds present in rich medium and decreased Cu²⁺ availability for CUP promoter induction.

The data in FIG. 6 demonstrate unambiguously that SUMO-Pro-GFP and CTHS-GFP fusions were not cleaved whereas SUMO-Met-GFP was efficiently cleaved in yeast. The proteins migrate at expected rate on SDS-PAGE and, noticeably, no partial cleavage is detected.

Thus, the foregoing example provides evidence showing that: (a) CTHS can enhance the expression of GFP, and (b) CTHS-GFP can be produced as an uncleaved fusion in yeast. Production of the fusions in eukaryotes as uncleaved entities offers tremendous advantages such as: (1) an attached affinity or epitope tag would remain attached to the expressed protein of interest, (2) such tags can be removed by a SUMO protease (see EXAMPLE III), and (3) certain proteins produced in E. coli are not active owing to absence of posttranslational modifications that can only be introduced in eukaryotes, and not bacteria.

EXAMPLE III Reconstitution of Cleavable Structure on the CTHS-fusion in Vitro and Cleavage by Ulp1

The molecular basis underlying the reconstitution of cleavable structure is outlined in FIGS. 3 and 4. A protein purified as a fusion with CTHS when mixed with NTHS can assemble into SUMO-like structure. In the next step this SUMO-like structure is recognized by a specific hydrolase which cleaves and releases the protein of interest, optionally with a new amino-terminus as described hereinabove. The following characteristics, among others, demonstrate the superiority of the instant invention:

1) Unlike fusions with full-length SUMO or other full-length Ubls, CTHS-fusion proteins are not cleaved in either prokaryotic or eukaryotic cells (demonstrated in EXAMPLE II).

2) CTHS-fusion and NTHS can assemble in vitro into a structure that is recognized and cleaved by specific hydrolase (see experiments below and FIGS. 7-9).

3) SUMO hydrolases are highly specific and extremely durable enzymes that are exceptionally well suited for in vitro bioprocessing applications.

FIG. 7A depicts the experimental results obtained when combining NTHS and CTHS-GFP in solution, resulting in assembly and formation of a cleavable complex. 6xHis-CTHS-GFP fusion protein purified from E. coli was incubated with SUMO hydrolase Ulp1 and 6xHis-NTHS (lanes 46), with 6xHis-NTHS (lanes 1-3) or Ulp1 (lanes 7-9). Release of free GFP can be observed only when all the components are present in the reaction mix. FIG. 7B demonstrates that the boundaries of NTHS and CTHS are not fixed and the functional elements of the system (e.g. CTHS and NTHS) may be changed without detectable effect on cleavage performance. Furthermore, purification tags such as, but not limited to 6xHis, GST, MBP, a sequence facilitating covalent coupling to solid support such as Cys residue, may be attached to either the amino- and/or carboxy-termini of NTHS.

The experiment depicted in FIG. 8 demonstrates that the fusion proteins are not cleaved in yeast. 6xHis-CTHS-GFP fusion was partially purified from yeast cells transformed with YEp-6xHis-CTHS-GFP (same as FIG. 6, lane 3). The protein was incubated with Ulp1 alone or with Ulp1 in the presence of NTHS. Once again the release of free GFP can be observed only when all the components are present in the reaction mix.

The experiment in FIG. 9 demonstrates that the technology is adaptable to other fusions. Namely, CTHS-GFP can carry a large affinity tag such as GST (glutathione transferase) to be used for the production and purification of the fusion proteins. Purified GST-CTHS-GFP fusion protein was incubated with purified NTHS and SUMO hydrolase Ulp1 (lanes 4 and 5) or in the absence of one of the components (lanes 2 and 3). When all the components are present in the reaction mix free GFP can be efficiently released. Thus, there is no hindrance (e.g., steric hindrance) to the reconstitution of SUMO or cleavage by Ulp1 when a large tag, such as GST, is employed.

Thus the instant invention allows for the expression of a protein of interest in a eukaryotic cell and the subsequent purification of the protein of interest. Notably, certain proteins produced in E. coli are not active owing to absence of posttranslational modifications which only occur in eukaryotes.

EXAMPLE IV Purification of CTHS-fusion Proteins by Affinity Chromatography on Immobilized NTHS

In addition to employing affinity tags such as, but not limited to 6xHis, GST, MBP, Flag and HA epitopes, fused to the amino-terminus of CTHS, the current invention provides yet another method of purifying CTHS. Specifically, CTHS can be purified by exploiting the natural affinity between CTHS and NTHS. Similarly, N-terminal domains of Ubls are expected to possess an affinity for the C-terminal domains of respective Ubls. NTHS (or N-terminal domains of other ub-like proteins) can be attached (covalently or noncovalently) to a solid support such as a chromatographic support. Additionally, a sequence facilitating covalent coupling to a solid support, such as a Cys residue or purification tag, may be attached to either the amino-or carboxy-terminus of NTHS. Bringing a solution (e.g., cell lysate) that contains a CTHS-protein of interest (also referred to as a passenger protein) fusion protein in contact with a solid support containing immobilized NTHS will allow affinity purification of the CTHS-protein of interest fusion protein. Following purification, the protein of interest is efficiently and faithfully cleaved from the junction by specific proteases.

The data presented in FIG. 10 provides experimental evidence for the feasibility of affinity purification of CTHS fusions on immobilized NTHS. Total soluble proteins (1 mg) from E. coli expressing the 6xHis-CTHS-GFP fusion protein were incubated with 15 μl (bed volume) of resins onto which specific proteins were covalently coupled. The resins were generated as follows. NTHS was cloned with an engineered cysteine residue at the N-terminus (Cys-6xHis-NTHS). Cys-6xHis-NTHS was expressed, purified, and coupled to Sulfo-link resin (Pierce; Rockford, Ill.). Similarly treated and blocked resin (empty resin) was obtained by omitting the Cys-6xHis-NTHS from the coupling reaction. Reduced BSA (lane 4) was coupled in the same manner as Cys-6xHis-NTHS. The IgG-immobilized resin was generated as follows. The total rabbit IgG fraction was purified from the sera of rabbits immunized with SUMO (Rockland immunochemicals; Gilbertsville, Pa.). IgGs were then coupled to Glyoxal-activated beads (Active Motif; Carlsbad, Calif.).

After a 1 hour incubation of the proteins with resin with constant mixing at room temperature, the resins were sedimented by mild centrifugation and washed 4 times with increasing concentrations of PBS. Washed resins were mixed with 30 μl of SDS-PAGE sample buffer and boiled for 2 minutes. 10 μl of each sample were resolved by 12% SDS-PAGE and stained with Coomassie. The significant presence of CTHS-GFP on the resin to which NTHS was immobilized, suggests a specific interaction between CTHS and NTHS and an approach for purification of CTHS fusions.

Importantly, the purification of CTHS-fusion proteins on immobilized NTHS will yield reconstituted cleavable SUMO structure. Such structures, however, can be reliably protected from cleavage by SUMO proteases naturally present in eukaryotic cells by addition of low concentrations (˜2 mM) of inexpensive inhibitors. Examples of such inhibitors include, without limitation, salts of zinc, cobalt and other metals. After purification of CTHS fusion proteins, these inhibitors are efficiently chelated by EDTA to allow the cleavage to proceed.

FIG. 10 also demonstrates that IgGs produced against SUMO can also be used as an affinity matrix when coupled to a solid support. CTHS fusions purified in this manner may be contacted with NTHS to generate reconstituted SUMO for cleavage and release of the protein of interest.

EXAMPLE V Modification of Amino Acid Sequences of NTHS and CTHS for Increased Affinity and Enhanced Purification Efficiency

The affinity between NTHS and CTHS can further be enhanced by introducing mutations into the sequences of NTHS, CTHS or both. A variety of mutations can be contemplated through a rational approach as the detailed 3D structures of SUMO, Ub, Rub1 and other Ubls are available. Directed mutagenesis may enhance the interactions between CTHS and NTHS due, for example, to the introduction of amino acids with higher hydrophobicity. For example, Lys⁴¹ (located in KIKK⁴¹TTPL (SEQ ID NO: 87) context of NTHS), which appears to be in close vicinity of Leu⁸¹ (located in EDL⁸¹DME (SEQ IS NO: 88) context of CTHS) may be replaced with a hydrophobic amino acid such as methionine. Such a substitution most likely will not affect the other properties of NTHS, such as the ability to serve as a substrate for Ulp1 when a part of reconstitutes SUMO, because the same positions in human SUMO-1 is occupied by methionine.

Alternatively, charged residues may be introduced in CTHS and NTHS. For example, Ile⁸⁸ (located in DNDI⁸⁸EAH (SEQ ID NO: 89) context of CTHS), which appears to be in a close vicinity of Lys²⁷ (located in INLK²⁷VSD (SEQ ID NO: 90) context of NTHS) may be replaced with a negatively charged amino acid such as Glu⁸⁸.

In addition to directed mutagenesis, random mutagenesis of CTHS or NTHS can be performed and the mutant molecules can be screened for binding affinities and those that show stronger affinities can be selected and used for expression and affinity purification.

Another method to increase the affinity between NTHS and CTHS consists of the addition of new amino acids at the C-terminus of NTHS and at the N-terminus of CTHS (indicated as the “boundary” in FIG. 3). For example, the addition of several positively charged amino acid residues (e.g. Lys or Arg) to the C-terminus of NTHS and the simultaneous addition of several negatively charged amino acid residues (e.g. Glu or Asp) to the N-terminus of CTHS, will result in the formation of multiple salt bridges between the oppositely charged amino acid residues and will stabilize the interaction between CTHS and NTHS. Other structures, peptides, or proteins known to interact strongly such as, but not limited to, anti-parallel β-sheets, anti-parallel α-helices (e.g. leucine zippers or coiled coils), and proteins that from dimers may also be used as an alternative to charged amino acids.

Another aspect of the instant invention involves the use of CTHS as an affinity tag to identify a small molecule that tightly binds the exposed surface on CTHS that normally interacts with NTHS (NTHS interacting CTHS surface). Identification of such a molecule may be conducted through computer based screening of atomic coordinates of library of chemical compounds versus the predicted atomic coordinates of NTHS interacting CTHS surface (in silico). Alternatively, CTHS and SUMO may be screened against the libraries of compounds immobilized on chromatographic supports (peptidomimetic approach). Identification of the candidate small molecules that selectively bind CTHS and not SUMO will indicate an interaction of the compound with NTHS interacting CTHS surface. Identified compounds may then be used to selectively purify CTHS fusion proteins but not SUMOylated proteins and free SUMO that are naturally present in eukaryotes. Chemicals identified by either in silico or conventional approaches may further be modified to provide new structures suitable for use as affinity ligand for chromatographic separations.

EXAMPLE VI Conventional Affinity Purification of CTHS Fusion Proteins and Cleavage with SUMO Hydrolase

FIGS. 4A and 4B present a diagram and flow chart that depict certain embodiments of the purification and cleavage of CTHS fusion proteins. This example explains these embodiments in more detail. The following tables list the solutions that can be used for the affinity purification of CTHS fusions that carry various affinity tags and their subsequent cleavage. TABLE 3 Solution Components Solutions for purification of 6xHis-CTHS fusions Lysis buffer 25 mM Tris pH 8.0; 50 mM NaCl Column loading buffer, 25 mM Tris pH 8.0; 250 mM NaCl; 10 mM Dialysis or Gel filtration imidazole buffers Wash Buffer 25 mM imidazole; 50 mM Tris pH 8.0; 250 mM NaCl; (optional) 5-10 mM β-mercaptoethanol (protein dependent) Elution Buffer 300 mM imidazole; 50 mM Tris pH 8.0; 250 mM NaCl; (optional) 5-10 mM β-mercaptoethanol (protein dependent) Solutions for purification of GST-CTHS fusions Lysis buffer 25 mM Tris pH 8.0; 50 mM NaCl Column loading buffer 25 mM Tris pH 8.0; 250 mM NaCl; Dialysis or Gel filtration buffers Wash Buffer 50 mM Tris pH 8.0; 250 mM NaCl; (optional) 5-10 mM β-mercaptoethanol (protein dependent) Elution Buffer 50 mM glutathion (reduced); 50 mM Tris pH 8.0; 250 mM NaCl; (optional) 5-10 mM β-mercaptoethanol (protein dependent)

From typical 250 ml cultures, the samples are pelleted by centrifugation, and supernatants are removed by decanting. Generally, from 250 ml of culture, 1.0-1.5 grams of wet cells are produced. Pelleted cells are then resuspended in 5-10 ml of lysis buffer. Samples are kept on ice throughout the sonication procedure. Using an appropriate tip, the samples are sonicated 3-5 times (10 second pulses) at 50% duty cycle. Sonicated samples may be adjusted to 1% Triton X-100 and incubated at 4° C. for 30 minutes to allow better recovery of CTHS fusion. Lysed samples (in lysis solution) are loaded onto 1-ml columns. The columns are washed with 5 to 10 volumes of wash buffer. Columns are developed with 2.5 ml of elution buffer.

A typical yield from a 1 ml column is 5 mg of fusion protein. Two examples of methods by which SUMO hydrolase cleavage may be performed are: 1) performing cleavage in a dialysis bag with 5 mg of NTHS (equimolar concentration to the expected yield of protein) and 500 units of SUMO protease added, and incubating overnight at 4° C.; and 2) removing the elution agent by dialysis or desalting (gel filtration), and subsequently adding 5 mg of NTHS and 500 units of SUMO protease to the sample, and incubating at room temperature for 2 hr or at 4° C. overnight. Cleavage may be monitored by gel electrophoresis. The reaction products from the above cleavage methods are loaded onto a column packed with an appropriate affinity resin. The protein of interest is recovered in the flow through, free of NTHS, CTHS and Ulp1. As described hereinabove, the cleavage reaction may also be performed directly on the column. A unit of SUMO protease (Ulp1) is defined as the amount of enzyme that cleaves 15 μg of pure SUMO-Met-GFP (up to 95%) in 50 mM Tris-HCl pH 8.0, 0.5 mM DTT, 150 mM NaCl at room temperature in 60 minutes.

After cleavage, protein of interest can be stored, directly used in intended applications, or subjected to additional purification steps. Removal of elution agent (by dialysis or desalting) and passing the sample through the resin that initially was used for purification of the CTHS fusion will trap most of the contaminating proteins resulting in 95% purity of the protein of interest.

The methods described here and shown in FIG. 4 primarily intend the use of identical affinity tags on all three protein components (e.g. tag-NTHS, tag-CTHS-PassengerProtein, tag-Ulp1). However, depending on the application, the tags may not necessarily be identical and all three components need not have a tag. Furthermore, two or more tags may be combined on the same protein (see for example, GST-6xHis-CTHS-GFP, presented in FIG. 9). Use of two tags may be beneficial in accelerated purification protocols in the following way: (1) After purification of GST-6xHis-CTHS-GFP on GSH resin and elution with GSH; (2) the fusion is brought in contact with 6xHis-NTHS and 6xHis-Ulp1 and incubated to allow cleavage; (3) the mix is directly applied onto Ni-resin column to subtract GST-6xHis, 6xHis-NHTS and 6xHis-Ulp1; and (4) GOP or the protein of interest is recovered in the flow through. Optionally, the GST-6xHis-CTHS-GFP construct may not contain the 6xHis tag and be retained on the Ni resin by its affinity for 6xHis-NTHS. These methods preclude the necessity to remove eluting agent prior to the subtraction step.

EXAMPLE VII Adaptation of Other Ubiquitin-like Proteins (Ubls) for Use with the Described Technology

The examples presented above demonstrate usefulness of partial SUMO sequences (CTHS and NTHS) for expression, purification and release of the protein of interest. In principle, any Ubl including ubiquitin (for examples see Table 1 and FIG. 1) may be utilized in a similar fashion. This example describes how other Ubl halves or domains may be generated and used in similar applications. FIG. 3 shows a 3-D structure of SUMO and the approximate position of the boundary between NTHS and CTHS. FIGS. 14, 15 and 25 give exact boundary positions and the sequences of two versions of CTHS and NTHS. The boundary between CTHS and NTHS was either introduced as in FIG. 14 or as in FIG. 25. It is important to note that when version 1 and version 2 of CTHS and NTHS were matched, the efficient cleavage was still achieved (see FIG. 7B). FIG. 26 gives examples of N-terminal domains (NTDs) and carboxy-terminal domains (CTDs) for each Ubl listed in Table 1. Two versions for each NTD and CTD are given. NTDs and CTDs for other Ubls (i.e. not listed in FIG. 26) can be deduced and utilized as follows. (1) The amino-acid sequence of the Ubl is aligned with the amino-acid sequences of SUMO (FIG. 14 or 25) or other Ubls (FIG. 26) either manually or using specialized software (e.g., pairwise alignment at www.ncbi.nlm.nih.gov/BLAST). It is sufficient to perform the alignment with just one other Ubl which is known to be closely related (e.g. Rub1 may be aligned with Nedd8); (2) The boundaries are determined with respect to the examples given in FIG. 26; (3) Respective coding DNA sequences are deduced from the sequence of the gene; (4) Cloning of the sequences encoding CTD and NTD is performed by standard methods as described above; (5) CTD is operably linked to the coding sequence of the protein of interest as described above; (6) proteins are expressed and purified as above; and (7) reconstitution of cleavable structure and cleavage by a specific hydrolase (e.g. DEN-1 if CTD and NTD of Rub1 is used; see FIG. 1) as described above.

EXAMPLE VIII Adaptation of Other Ubiquitin Fold Proteins (Ubfs) for Use with the Described Technology

Example VII provides information on how partial sequences of any Ubl, including SUMO and ubiquitin (for examples see Table 1 and FIG. 1), may be utilized for expression, purification and cleavage of proteins of interest. It is important to emphasize that structurally Ubls are not unique but have easily traceable structural similarities to some other proteins which may not be evolutionary or functionally related and may have minimal or no sequence similarity (44). These proteins may be classified as having a Ubiquitin fold (also referred to as beta-Grasp fold or beta-Grasp domain) (24, 54-59). Examples of Ubiquitin fold proteins (Ubfs) are provided in Table 4 (see also references 54-59). The structural feature that separates these proteins from others and places them in a specific class is a sequence of β-sheets and an α-helix (β-β-α-β-β) in the molecule that folds into a structure with specific positioning of the β-sheets in the order 2143 (25). Interestingly, the first two β-sheets (2 and 1) are anti-parallel as well as the two last ones (4 and 3), whereas the first and the last ones (1 and 4) are parallel. This arrangement may be referred to as β2, antiparallel-β1, parallel-β4, antiparallel-β3. Slight variations to the ubiquitin fold can be observed. The lengths of structural determinants (β-sheets and α-helix) and connecting loops may be different. Some proteins may contain additional β-sheets or α-helices (e.g., Ubls possess an extra strand in the β-sheet and a very short helix in the loop), yet the overall fold (β2, antiparallel-β1, parallel-β4, antiparallel-β3) is easily and unambiguously recognized. Yearly updates on Ubf family are found at the SCOP database (Structural Classification of Proteins) at the following URL (scop.mrc-lmb.cam.ac.ukscop/data/scop.b.e.bi.html).

A careful analysis of the body of literature available on proteins that contain ubiquitin fold beta-Grasp domain) as part of their sequence reveals that all of the domains are easily produced as recombinant proteins. Furthermore, some data demonstrate that these domains serve as natural chaperones that stabilize a large protein that contains such beta-Grasp domain. It is therefore our conclusion that beta-Grasp fold, either in its entirety or as a part thereof (e.g. CTHS), may be successfully used as a fusion partner for enhancement of expression, stability and solubility of C-terminally fused recombinant proteins. Importantly, all the methods described hereinabove can be directly used with any of the Ubfs. Additionally, the NTD of any Ubf may be expressed and immobilized on solid support and further used as an affinity matrix for purification of CTD fused with the protein of interest.

Methods are provided herein for employing Ubfs as tags that enhance expression, stability and solubility of the fused proteins and that NTD of said Ubf as a part of an affinity matrix for purification of the fusion of CTD of said Ubf with the protein of interest. FIG. 3 shows a 3-D structure of SUMO and the approximate position of the boundary between NTHS and CTHS. FIGS. 14, 15 and 25 give exact boundary positions and the sequences of two versions of CTHS and NTHS. The boundary between CTHS and NTHS was either introduced as in FIG. 14 or as in FIG. 25. FIG. 26 gives examples of NTDs and CTDs for each Ubl listed in Table 1. FIG. 27 gives examples of NTDs and CTDs for some Ufds listed in Table 4. Two versions for each NTD and CTD are given. NTDs and CTDs for other Ufds (i.e., not listed in FIG. 27) can be deduced and utilized as follows. (1) The amino-acid sequence of the Ufd (ubiquitin fold domain) is aligned with amino-acid sequences of a homologous Ufd found in FIG. 27 either manually or using specialized software (e.g., pairwise alignment at www.ncbi.nlm.nih.gov/BLAST). It is sufficient to perform the alignment with just one other Ufd that is known to be closely related (e.g. CIDA may be aligned with CPAN); (2) The boundaries are determined with respect to the examples given in FIG. 27; (3) Respective coding DNA sequences are deduced from the sequence of the gene; (4) Cloning of the sequences encoding CTD and NTD is performed by standard methods as described above; (5) CTD is operably linked to the coding sequence of the protein of interest as described above; (6) a recognition site for specific protease (e.g. multibasic cleavage site such as RvRR and recognition sequences of thrombin, Factor Xa, enterokinase, and Tev) may be engineered into the construct following the CID and before the protease; (7) proteins are expressed and purified as above; and (8) cleavage is performed by the specific protease and the reaction products are purified as described hereinabove. Notably, because the cleavage site may be a movable entity in this aspect of the invention, the fusion protein may, alternatively, comprise the NTD, the cleavage site, and the protein of interest and be purified by immobilized CID.

If, based on its solved secondary structure, a new protein or its part is classified as having a ubiquitin fold, then the boundaries of the sequence suitable for use as a fusion tag can be determined using the following criteria: (1) full length tag must contain the sequences coding for two β-sheets, an α-helix and two β-sheets (β2, antiparallel-β1, parallel-β4, antiparallel-β3); (2) NTD must contain two anti-parallel β-sheets and an α-helix (β-β-α-, however, the α-helix may not necessarily be full-length) with or without the loop that connects the α-helix with the following β-sheet; and (3) CTD must contain two antiparallel β-sheets, whether or not interrupted by an extra sheet and/or helix (however, the first β-sheet may not necessarily be full-length). TABLE 4 Properties of Ubiquitin-fold Proteins (Ufds) Domain or domain Function Simi- class of Ubf Function of larity name domain the protein Examples to Ub ACC Nos. UDP, Interaction Ubiquitin RAD23, 20-80% NP_010877 Ubx, with ligases, signal Parkin, XP_011437 UBQ, UBA, proteasome, transduction Dsk2 NP_014003 etc. chaperone Fbx7 CAD and Protein- Caspase Bemlp, CPAN None AAC39709 PB1 protein activated DEF-40, 1IP9_A interaction DNAse, CIDE-B signaling MoaD/ThiS Sulfide Regulation of MoaD None NP_752796 transfer mitosis. ThiS Tgs-like unknown Threonyl- ThrRS None NP_288153 tRNA synthetase Ferredoxin/ Electron Electron Ferredoxin, None NP_442127 ferredoxin- transport transport Xanthine NP_355267 like oxidase Staphylokinase/ Unknown Plasminogen Staphylokinase None NP_375053 Streptokinase activator Superantigen Unknown Cell surface Enterotoxin None NP_269186 toxins protein c2, superantigen Spe-C Immuno- Immuno- Cell surface Protein G None P06654 globulin globulin protein, binding binding IF3 Ribosome Translation Translation None P03000 binding initiation initiation factor IF3 Glutamine unknown Glutamine Glutamine None NP_458042 synthetase synthesis synthetase

EXAMPLE IX Examples of Various Classes of Proteins Whose Fusion with Carboxy-Terminal Half of SUMO (CTHS) Will Improve Quantity and Quality of Proteins

The design and construction of all the pET vectors expressing GPP has been described above. As with GFP, any DNA sequence can be cloned as a fusion with 6xHis-CTHS. Furthermore, the 6xHis-CTHS-protein of interest fusion protein can be recloned into any yeast (e.g., Yep), baculoviral (e.g., pFastBac), mammalian (e.g., pCDNA3) or other host vectors by, for example, a simple cut-and-paste procedure identical or similar to the one described above. Cell propagation, protein expression, harvesting, lysis, and purification of fusion are performed described above or according to one of the established procedures described elsewhere. Cleavage of fusions and subtraction of NTHS and Ulp1 is performed as described, for example, in Example III. The system described in this application has several major advantages as compared to other systems currently in use. One or more of these advantages may create a precedent for preferred use of the system described here over other systems. Industrial needs for protein manufacturing may suggest that the following classes or groups of proteins may be exemplified as the ones most likely to benefit from use of the current invention. Some of the factors that may be considered when expression system is selected are (1) the success of expression of the protein in various systems, (2) requirement for specific N-terminus, and (3) requirement for glycosylation. Each of these factors can be successfully addressed by the system described herein: (1)CTHS enhances the production of proteins, (2) robust cleavage by Ulp1 allows generation of any amino-terminus, and (3) expression in insect or mammalian cells will allow introduction of necessary post-translational modifications. Table 5 provides representative molecules and GenBank Accession numbers for the expression of the proteins listed using the methods of the invention. TABLE 5 Examples of Proteins and Protein Classes for use with current invention Success Require- Require- rate with ment for ment for conventional specific glycosy- Protein class systems Protein N-terimini lation ACC# Cytokine High/ IL-15 Yes No P40933 inclusion IFNγ P01579 bodies P01375 Chemokine High/ Myb-1β Yes No P13236 inclusion bodies Growth low TGFα Yes Varies P01135 factors Enzymes Low DNA Yes No 1RDR polymerase Very low DNase II alpha Yes Yes AAC77366 Very low Cathepsin Unknown Yes P07858 (peptidase) Very low BMP-1 protease Unknown Yes NP_001190 Peptides Low/ Calcitonin Yes varies CAA26189 including proteolysis therapeutic Nuclear high LXR No varies Q13133 receptors Cytokine/ Low/ IFNAR, No varies P15260 chemokine/ inclusion VEGFR, P35968 growth factor bodies CCR9 NP_034043 receptors P00533 Ser-Thr Low MAPK No No P49137 kinases Tyr kinases Extremely Zap 70 No No P43403 low/ inactive Transcription low CREB Unknown Yes NP_005185 factors Initiation low eIF2 Unknown Yes NP_004085 factors Viral/parasite/ Low Spike protein, Varies varies AAP33697 bacterial Apa (Rv1860), Q50906 proteins RAP-1 AAF15365 (vaccines)

EXAMPLE X Use of Carboxy-Terminal Half of SUMO (CTHS) for Enhanced Secretion of the Proteins in E. coli, Yeast, Insect and Mammalian Cells

The basic characteristics of CTHS (e.g. small size and compact structure), should allow the use of CTHS in combination with secretion signals. The design and construction of all the vectors described above may be employed to allow a suitable secretion signal to be operably linked to the N-terminus of CTHS and affinity tag (see Table 6). TABLE 6 Examples of secretion signals and construct configurations Secretion Host signal Fusion configurations Ref. Bacterial E. coli PelB PelB-AffinityTag-CTHS-PassengerProtein (45) PelB-CTHS-PassengerProtein (periplasmic localization and release) Bacterial RsaA RsaA-AffinityTag-CTHS-PassengerProtein (46) Caulobacter RsaA-CTHS-PassengerProtein crescentus (true secretion) Yeast α-factor α-AffinityTag-CTHS-PassengerProtein (47) Saccharomyces α-CTHS-PassengerProtein cerevisiae α-AffinityTag-CTHS-PassengerProtein (48) Yeast α-CTHS-PassengerProtein Pichia paseoris PHO PHO-AffinityTag-CTHS-PassengerProtein (49) Yeast PHO-CTHS-PassengerProtein Saccharomyces pombe Insect cells gp67 gp67-AffinityTag-CTHS-PassengerProtein (50) Sf9 gp67-CTHS-PassengerProtein Insect cells Hbm Hbm-AffinityTag-CTHS-PassengerProtein (51) HY5 (honey Hbm-CTHS-PassengerProtein Insect cells bee Drosophila mellitin) melanogaster Mammalian IgK IgK-AffinityTag-CTHS-PassengerProtein (52) cells IgK-CTHS-PassengerProtein CHO Mammalian cells 293 E. coli can not efficiently express and secrete mid-size and large proteins and, therefore, is not a preferred organism. However, certain peptides can be efficiently expressed and secreted from E. coli in very large quantities (53) and it is possible that fusions with CTHS will further improve yield and quantity of the peptides. More frequently used systems for extracellular production of proteins in prokaryotes include Bacillus brevis, Caulobacter crescentus and several others. Other hosts (yeast, insect and mammalian cells) are also used to produce proteins extracellularly. The secretion involves endoplasmic reticulum mediated pathway that ensures that only properly folded proteins are secreted. Therefore, at least in some cases, extracellular production of proteins may me advantageous because (1) the starting material will contain less contaminated proteins and non-proteinacious substances and (2) most of the protein will be properly folded. Upon capturing, and, if needed, further purification, CTHS is conveniently cleaved by Ulp1 and subtracted using appropriate affinity support.

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While certain of the preferred embodiments of the present invention have been described and specifically exemplified above, it is not intended that the invention be limited to such embodiments. Various modifications may be made thereto without departing from the scope and spirit of the present invention, as set forth in the following claims. 

1. A method for enhancing the expression a protein of interest in a host cell comprising: a) providing a nucleic acid construct which encodes a fusion protein wherein said construct contains i) a nucleic acid sequence encoding a carboxy-terminal domain of a ubiquitin-like (Ubl) molecule operably linked to ii) a nucleic acid sequence encoding said protein of interest; and b) expressing said nucleic acid construct in said host cell, whereby the presence of said carboxy-terminal domain of a ubiquitin-like (Ubl) molecule in said fusion protein increases the expression level of said protein of interest in said host cell.
 2. The method of claim 1, wherein said ubiquitin-like (Ubl) molecule is selected from the group consisting of: SUMO, ubiquitin, RUB, HUB, APG8, APG12, URM1, FAT10, Ubl-L and ISG15.
 3. The method of claim 2, wherein said ubiquitin-like (Ubl) molecule is SUMO.
 4. The method of claim 1, wherein said nucleic acid sequence encoding a carboxy-terminal domain of a ubiquitin-like (Ubl) molecule comprises a sequence selected from the group consisting of SEQ ID NO: 9 and SEQ ID NO:
 31. 5. The method of claim 1, wherein said host cell is selected from the group consisting of a yeast cell, E. coli, a bacterial cell, a mammalian cell, and an insect cell.
 6. The method of claim 1, wherein said nucleic acid construct encoding a fusion protein is in a vector.
 7. The method as claimed in claim 1, wherein the fusion protein, when expressed, comprises the carboxy-terminal domain of a ubiquitin-like (Ubl) molecule attached to the amino-terminus of the protein of interest such that cleavage site of the Ubl molecule is immediately amino terminal to the protein of interest, said method further comprising; c) contacting said expressed fusion protein with an amino-terminal domain of the same ubiquitin-like (Ubl) molecule, thereby generating a reconstituted Ubl; and d) contacting said reconstituted Ubl with a protease specific to said Ubl molecule, thereby cleaving said fusion protein such that said protein of interest is produced.
 8. The method of claim 7, wherein said amino terminal domain of said Ubl molecule of c) comprises a first purification tag, said method further comprising purifying said reconstituted Ubl generated in step c) on a solid support capable of specifically binding said first purification tag, prior to step d); and said method further comprising: e) purifying said protein of interest.
 9. The method of claim 8, wherein said first purification tag is selected from the group consisting of a polyhistidine tag (6xHis), a polyarginine tag, glutathione-S-transferase (GST), maltose binding protein (MBP), S-tag, influenza virus HA tag, thioredoxin, staphylococcal protein A tag, the FLAG™ epitope, AviTag epitope, and the c-myc epitope.
 10. The method of claim 7, wherein said fusion protein is purified prior to contacting said amino-terminal domain of the same ubiquitin-like (Ubl) molecule by immuoprecipitation with an antibody specific to a protein selected from the group consisting of SUMO (SEQ ID NO: 1) and CTHS (SEQ ID NO: 9).
 11. The method of claim 8, wherein said fusion protein comprises second purification tag attached to the amino-terminus of the carboxy-terminal domain of a ubiquitin-like (Ubl) molecule, said method further comprising purifying said fusion protein expressed in step b) on a solid support capable of specifically binding at least one second purification tag, prior to said contacting with said amino-terminal domain of said Ubl molecule in c).
 12. The method of claim 11, wherein said at least one second purification tag and said second purification tag is selected from the group consisting of a polyhistidine tag (6xHis), a polyarginine tag, glutathione-S-transferase (GST), maltose binding protein (MBP), S-tag, influenza virus HA tag, thioredoxin, staphylococcal protein A tag, the FLAG™ epitope, AviTag epitope, and the c-myc epitope.
 13. The method of claim 11, wherein said at least one second purification tag and said first purification tag are the same.
 14. The method of claim 11, wherein said at least one second purification tag and said first purification tag are different.
 15. The method of claim 8, wherein said amino-terminal domain of the same ubiquitin-like (Ubl) molecule is attached to said solid support capable of specifically binding said first purification tag prior to contacting said purified fusion protein.
 16. The method of claim 11, wherein said at least one second purification tag is more than one and said second purification tags are different.
 17. The method of claim 15, wherein said first purification tag is a cysteine residue and said solid support possesses a thiol-reactive group.
 18. The method of claim 7, wherein said protease specific to said Ubl molecule is Ulp1.
 19. The method of claim 7, wherein said protease specific to said Ubl molecule further comprises a third purification tag.
 20. The method of claim 19, wherein said third purification is the same as said first purification tag.
 21. The method of claim 19, wherein said third purification tag is different than said first purification tag.
 22. The method of claim 19, further comprising contacting said specific protease to said Ubl molecule with a solid support capable of binding said third purification tag, thereby removing said specific protease from said protein of interest and thereby further purifying said protein of interest.
 23. A method of increasing the affinity between a carboxy-terminal domain of a ubiquitin-like (Ubl) molecule and a amino-terminal domain of a ubiquitin-like (Ubl) molecule comprising operably linking a moiety to said carboxy-terminal domain and to said amino-terminal domain.
 24. The method of claim 23, wherein said carboxy-terminal domain of a ubiquitin-like (Ubl) molecule and said amino-terminal domain of a ubiquitin-like (Ubl) molecule are CTHS and NTHS, respectively.
 25. The method of claim 23, wherein said moiety attached to the carboxy-terminal domain of a ubiquitin-like (Ubl) molecule and said moiety attached to the amino-terminal domain of a ubiquitin-like (Ubl) molecule comprise an anti-parallel β-sheet structure.
 26. The method of claim 23, wherein said moiety attached to the carboxy-terminal domain of a ubiquitin-like (Ubl) molecule and said moiety attached to the amino-terminal domain of a ubiquitin-like (Ubl) molecule comprise an anti-parallel α-helix structure.
 27. The method of claim 23, wherein said moiety attached to the carboxy-terminal domain of a ubiquitin-like (Ubl) molecule comprises negatively charged amino acids and said moiety attached to the amino-terminal domain of a ubiquitin-like (Ubl) molecule comprises positively charged amino acids.
 28. The method of claim 23, wherein said moiety attached to the carboxy-terminal domain of a ubiquitin-like (Ubl) molecule comprises positively charged amino acids and said moiety attached to the amino-terminal domain of a ubiquitin-like (Ubl) molecule comprises negatively charged amino acids.
 29. A method of increasing the affinity between a carboxy-terminal domain of a ubiquitin-like (Ubl) molecule and a amino-terminal domain of a ubiquitin-like (Ubl) molecule comprising inserting mutations into said carboxy-terminal domain and said amino-terminal domain.
 30. The method of claim 29, wherein said mutations increase the hydrophobicity between said carboxy-terminal domain and said amino-terminal domain.
 31. The method of claim 30, wherein Lys⁴¹is substituted with a hydrophobic amino acid.
 32. The method of claim 29, wherein said mutations introduce charged amino acids.
 33. The method of claim 32, wherein Ile⁸⁸is substituted with a negatively charged amino acid.
 34. The method of claim 8, further comprising an inhibitor of said protease specific to said Ubl molecule during step c).
 35. The method of claim 34, wherein said protease inhibitor is selected a salt of heavy metal.
 36. The method of claim 35, wherein said heavy metal is selected from the group consisting of zinc and cobalt.
 37. The method of claim 34, further comprising removing said protease inhibitor with a metal ion chelator.
 38. The method of claim 37, wherein said metal ion chelator is EDTA.
 39. The method of claim 8, further comprising eluting said reconstituted Ubl from the solid support prior to contacting with said specific protease.
 40. The method of claim 39, wherein said reconstituted Ubl is eluted by the addition of excess said amino terminal domain of a Ubl molecule or a derivative thereof.
 41. The method of claim 39, wherein said reconstituted Ubl is eluted by changing a characterisitic of the solvent selected from the group consisting of pH, salt concentration, chaotropic status, and polarity.
 42. A kit comprising a recombinant vector containing a nucleic acid sequence encoding a carboxy-terminal domain of a ubiquitin-like (Ubl) molecule, wherein said Ubl molecule is selected from the group of SUMO, ubiquitin, RUB, HUB, APG8, APG12, URM1, FAT10, Ubi-L and ISG15, operably linked to a promoter suitable for expression in the desired host cell and a multiple cloning site suitable for cloning a nucleic acid encoding the protein of interest in-frame with the nucleic acid sequence encoding the UBL molecule.
 43. The kit of claim 42, wherein said kit further comprises host cells suitable for expression of said vector.
 44. The kit of claim 42, wherein said host cells are selected from the group of yeast cells, E. coli, insect cells, and mammalian cells.
 45. The kit of claim 42, wherein said kit further comprises reagents for altering the nucleic acid encoding said protein of interest to generate amino termini which are different from those native to the wild-type protein.
 46. The kit of claim 45, wherein said kit comprises reagents suitable for site-directed mutagenesis.
 47. The kit of claim 46, wherein said kit comprises oligonucleotides for performing oligonucleotide-based site-directed mutagenesis.
 48. The kit as claimed in claim 42, further comprising a protease composition capable of cleaving a reconstituted Ubl molecule from the fusion protein.
 49. (canceled)
 50. (canceled)
 51. The kit of claim 48 further comprising at least one item selected from the group consisting of: a) at least one solid support for binding the purification tag, b) lysis buffers, c) wash buffers, d) elution buffers, e) cleavage buffers, f) the amino terminal domain of said Ubl molecule in protein form, g) a nucleic acid encoding for the amino terminal domain of said Ubl molecule operably linked to second purification tag, and h) instruction material.
 52. A method for generating an altered amino terminus in a protein of interest in a host cell comprising; a) providing a nucleic acid sequence encoding said protein of interest; b) altering the N-terminal amino acid coding sequence in said nucleic acid; c) operably linking a nucleic acid encoding a carboxy-terminal domain of a ubiquitin-like (Ubl) molecule to said nucleic acid sequence, thereby producing a nucleic acid encoding a fusion protein wherein said carboxy-terminal domain of a ubiquitin-like (Ubl) molecule is at the amino terminus of the protein of interest; d) expressing said nucleic acid encoding said fusion protein in a eukaryotic cell; and e) expressing a nucleic acid molecule encoding a amino terminal domain of said Ubl molecule, thereby producing said protein of interest in said cell, wherein said protein of interest having an altered amino terminus. 